Secrets of Home Theater and High Fidelity
Volume 1, Number 1, 1994
Section 2. Compact Disc, Laserdisc, and DVD Players (Revised December, 2000)
Index for Compact Disc, Laserdisc, and DVD Players:
Digital Sampling Audio Bits Laserdisc Player Connectors Dolby AC-3 (DD) and DTS Multibit DACs vs. 1 Bit DACs Oversampling Digital to Analog Converter Jitter Digital Video Disc (DVD), SACD Upsampling High Definition Compatible Digital (HDCD) Playing Both Sides of Laserdiscs Laserdiscs LD and DVD Features
In the audio industry, nothing has revolutionized the way we listen to recorded music like the compact disc (CD). Commercial availability of compact discs and players is a little more than a decade old, and over this time, CDs have slowly replaced the Long Playing (LP) record to the point that music stores (except for the specialty market) carry only CDs and cassette tapes. LPs represent the last vestige of analog music reproduction, where the musical sound wave is recorded continuously.
With CDs, music is recorded in digital fashion. Instead of a continuous recording of the waveform, the music is sampled 44.1 thousand times per second (44.1 kilo Hertz or 44.1 kHz) for each of the two channels (left and right stereo). The information is stored on a spiral track, beginning near the center and winding its way to the outside edge. LPs are recorded in a spiral groove beginning at the outside edge and winding their way to the center. The difference between a CD and an LP is what is in the spiral. The spiral track of a stereo LP is a series of peaks and valleys engraved at 45 degree angles to the surface, one for the left channel and one for the right. In a CD, the spiral contains a series of small bumps (usually called "pits") and flat areas (called "land") in between and beside the bumps. A small laser beam is aimed onto the spiral as the disc spins, and light is reflected from an area about three times as wide as one of the bumps. When the beam is on a spot where there are no bumps, all the light is reflected together. However, when the beam reaches a bump, light is reflected from the land on either side of the bump, and from the bump itself. Since the bump is higher than the land, its reflected light is slightly out of synchronization (called being "out of phase") with the light being reflected from the land, and the total amount of light reflected is reduced because of partial cancellation. The reflected light is directed onto a sensor which reads the light as a series of "on" (when only reflected from land) or "off" (when reflected from a bump and surrounding land) "bits" of digital data. An on bit is transmitted or stored as a one (1), and an off bit is transmitted or stored as a zero (0). The process of encoding different portions of the waveform by digital words of a given number of bits is called "Pulse Code Modulation" or PCM. This means that the waveform (the modulating signal, i.e., the music) is represented by a set of discrete values. In the case of music CDs using 16 bit words, there are 216 word possibilities (65,536). A value within a given range is assigned one of the 216 16 bit words. PCM is a "coded modulation", whereas the video and two analog channels on laserdiscs are represented by a noncoded modulation, called "Pulse Width Modulation" or PWM. In this case (PWM), the values can be any number, defined by the width of the pulse (the length of the pit in the track). Because the pit can represent any number, rather than one of a discrete set of numbers, PWM signals are analog. The PCM tracks in CDs are represented by 216 values, and hence, are digital. Furthermore, for current CDs, the bits are encoded onto the disc in what is called "EFM" or "Eight-to-Fourteen Modulation", meaning that an 8 bit packet of data is "Modulated" to 14 bits. An original 8 bit segment such as 0 0 0 0 0 0 1 1 would have the following code in EFM: 0 0 1 1 1 0 0 0 0 0 0 0 0 0, and 1 1 1 1 0 0 1 0 would have this code: 0 0 0 0 0 0 1 1 1 1 0 0 0 1. The coding is part of the "Overhead" that allows the process to run smoothly. When the disc is played, the bits are read as a square wave, put onto an AC waveform so that a 0 is represented by 2 volts, while a 1 is represented by 3 volts. Then the AC waveform is extracted, and the overhead bits removed, so that the "music" bits can be presented to the Digital-to-Analog Converter (DAC) as 1s and 0s after the process of "Demodulation". The DAC converts the 1s and 0s to the analog signal (which is fed to the CD player's output jacks for connection to an amplifier).
One stream of 16 on or off bits constitutes one of the samples (called a word, where, in the case of CDs, the word length is 16 bits) of musical information collected 44.1 thousand times per second as mentioned above. First, 16 bits are read for one channel (for example, the left channel), and then 16 bits are read for the other channel. Only about one third of the 4.3218 million bits per second read from a compact disc represent the audio signal, and the bitstream is controlled by a master clock of that same frequency (4.3218 Mega Hertz, or MHz). The rest are used for "Data Management" or "Overhead". The order of the bits in terms of whether each bit is on (1) or off (0) is a code for one tiny spot (in the form of voltage) on the musical waveform. For example, a word might be represented by the sequence 1001101000101001 (called a binary number).
In a way, it is like Morse Code, where each unique series of dots and dashes is a code for a letter of the alphabet. There are 216 (more than 65,000) possible combinations of 16 bits being on or off in sequence, allowing CD recordings very fine detail as to how the waveform is coded. Even with 65,000 combinations of waveform coding (32,500 for the positive part of the waveform and 32,500 for the negative), the sound level (voltage) may occur in between any two sequential points in the 65,000 combinations. The player must round the value up or down to the nearest value in the 65,000 possible combinations, and this is called "Quantization Error". The ability for a CD player to handle quantization error varies, and is occasionally used as a selling point. The next generation of digital discs, of which the CD was the first type, has 48 kHz up to 192 kHz sampling and 20 - 24 bit words. This is the Audio-Only DVDs (Digital Video Disc, or Digital Versatile Disc), called DVD-Audio or DVD-A, which has the room to hold more digital data (about 15 Gigabytes of data compared to 750 Megabytes for CDs). Keep in mind that DVDs are really just CDs at a higher technological capability, i.e., they hold more data. It is sort of like upgrading your computer hard disk drive from a 750 MB capacity to a 15 GB capacity.
Digital to Analog Converter
So, as the disc spins along, 44.1 thousand samples, each represented by a series of 16 "1" or "0" bits, are collected each for the left and right stereo channels. These samples are then sent to a Digital to Analog Converter (DAC). The job of the DAC is to connect the samples together end to end in order to produce a smooth representation of the musical waveform, sort of like connecting the dots to make a picture in the coloring books we used as children. However, instead of connecting the dots (samples) with straight lines, the DAC uses sophisticated algorithms (mathematical equations) to fit the samples into a curved waveform that is more representative of the way the music truly sounds. Early digital music sounded rather harsh, in part because the DAC had a hard time making accurate guesses about the curve of the waveform in between the samples. (The CDs themselves were not all that great either.) Modern CD players have corrected much of this problem, and CDs have improved as well.
An artifact called "jitter" remains a difficulty. Jitter occurs when the distance between digital information, in time, is incorrect. For example, when an "off" bit is followed by an "on" bit, and the "off" bit is read for too long a period of time, it crowds on to the time that the "on" bit would be normally read and confuses the interpretation. Think of it again as receiving Morse Code. If the sender is inconsistent in the length of time the key is held down to make dots and dashes, you might mistake a short dash for a dot, and a long dot for a dash.
Another way of looking at jitter is when the DAC receives a 16 bit word, a command is given, converting the 16 bit word to a voltage. Jitter causes the command to be given a little too early or a little late. It results in a distortion of the waveform, making it bulge outward if the bits are received too early, and curve inward if received too late.
This is a simplification of a very complicated factor, but in any case, it is the timing of the data stream that is involved where jitter is concerned. However, there are many engineers who feel jitter is not really an audible problem and dismiss it as unimportant. Newer machines are being designed with low jitter in mind. Although the sound quality is the ultimate basis for a final decision on whether to purchase a particular CD player, you can ask for the specification on this if it is available when considering your purchase, with the lower the jitter, the better (measured in picoseconds, or trillionths of a second, that's right, trillionths!) Jitter rates of 30 picoseconds or less are found in top quality CD players. CD players with high jitter can sound harsh and also lose fine detail, including the upper natural harmonics. One new technique is to store the digital data from the CD in a memory buffer where it can be reclocked and fed to the DAC in precise sequence. CDs can also differ in quality. Although CDs all have 16 bit words representing the audio, a poorly engineered CD might have 14 bits of music while the last 2 of the 16 bits in its audio data stream consist of noise coming from any of several sources. Also, many CDs simply do not make use of the highest voltage values, since there is a "brick wall" when the volume hits that 32,500th value. By keeping the recording level down, that brick wall is never reached.
Multibit DACs vs. 1 Bit DACs
Some CD players use multibit DACs (the bits are read in groups), and some use 1 bit DACs, where the bits are read one at a time. Having several DACs (one for each channel) is also becoming fashionable. Although 1 bit DACs are a modern trend, they are more susceptible to noise than multibit DACs. However, "noise shaping" helps to correct the problem. It is interesting to note that music could be coded with 1 bit words instead of 16 bit words, but the noise level would be quite unacceptable, in other words, terrible. In fact, the word length (number of bits per sample) determines the amount of noise (called signal to noise ratio).
The newest form of CDs, called DVD-Audio, or DVD-A, have 24 bit samples, which substantially improve this ratio. Manufacturers of some of the more expensive CD players claim they can "resolve" (read with accuracy) 24 bit samples, but the CDs themselves are still just 16 bits. Since many recordings are made with 20 bit word length recorders (converted to 16 bits for the CD itself), there is another process where the player attempts to recreate the 20 bit sound from the 16 bit words that are on the CD. This is called HDCD (see below). Also, Dolby Digital and DTS both have 20 bit word lengths.
Many audio engineers feel that a higher sampling rate would improve the sound, but at a great expense to the convenient size of current discs (5 inches, and 3 inches) which can easily be used not only in the home, but in automobiles, and portable units. By Nyquist's theorem, it was felt that you only need two samples to decode the maximum frequency in a waveform, and therefore, since the maximum audible frequency is 20 kHz, the sampling rate could be around 40 kHz. It ended up being 44.1 kHz, but now this number is considered too low. DVD-A remedies this problem because it can hold much more data. "Oversampling" is a process whereby the player reads two samples, and, using DSP, additional values are subsequently "interpolated" in between those two. Thus, if the CD player uses 8X oversampling, then the player creates 7 additional values in between each of the actual single samples on the disc (7 + 1 = 8, for 8X oversampling). Oversampling is for the purpose of removing artifacts when the signal is passed through an analog filter to the output. The DAC performance probably does improve. However, there is controversy over this procedure.
Because moderns DACs can handle 24 bit - 192 kHz samples, some players have a chip that "upsamples" the data, such as 16 bit - 44.1 kHz samples, to higher rates. The upsampler tries to "interpolate" what the samples would have been if the original music had been sampled at the higher rate. The results are often quite good, but of course, nothing is better than having the actual music samples. DVD-A (see below) has up to 24 bit - 192 kHz samples to begin with, and the sound quality is spectacular.
High Definition Compatible Digital (HDCD)
High Definition Compatible Digital (HDCD) is a technology developed by Pacific Microsonics, now owned by Microsoft. The method attempts to take a recording made at high sampling rate (greater than 100 kHz) and with long word length (20 - 24 bits), and then transcribe it onto a 44.1 kHz, 16 bit CD. Additional information is stored on the CD so that when the HDCD recording is played back on a HDCD player, some of the benefit of the high sampling rate and longer word length comes through. HDCD discs are said to sound better even on a regular CD player, and regular CDs are claimed to sound better on a HDCD player. There are many HDCD CDs available, and numerous players have HDCD capability. High Definition Compatible Digital might be confused with High Density Compact Disc (also abbreviated HDCD), but which is now called Digital Video Disc, Digital Versatile Disc, or DVD (see below).
Digital Video Disc or Digital Versatile Disc (DVD)
Digital Video Disc (DVD) used to be called High Density Compact Disc, but the abbreviation got confused with High Definition Compatible Digital (see above). Now it is called Digital Versatile Disc. DVD is a format released to consumers in the first quarter of 1997 (USA) which stores about 20 times as much information as a standard CD, and on the same size disc (5"). This allows several hours of movie (the DVD-Video or DVD-V) and multi-channel sound (such as AC-3 [Dolby Digital - DD] and DTS; see below) to be contained on a smaller disc than movies are on laserdiscs. Thousands of DVD movies have been released, with 5.1 DD surround sound. The audio and video are completely digital, rather than having some portions in analog form as is the case with laserdiscs. This produces home theater of profoundly improved quality over current broadcast TV, VHS tapes, and even laserdiscs.
Many DVDs are enhanced for 16:9 viewing. This means the image is stored in much the same way anamorphic films are stored. That is, the image is squeezed side-to-side. In order to use this feature, you must have a 16:9 television, which has the anamorphic mode. When playing a 16:9 enhanced DVD on a 16:9 TV in this mode, the final image occupies almost the entire viewing screen area rather than having so much of the screen taken up by blank bars at the top and bottom. This results in the same resolution side-to-side, but more resolution top-to-bottom, because more of the scanning lines are being used for the picture. A 16:9 TV can also just enlarge the image so that the widescreen movie fits into the screen area, but this discards the scanning lines above and below the picture, so it is not as high resolution as the 16:9 enhanced image. Not all manufacturers have jumped into the enhanced DVD production, because most consumers don't have 16:9 TVs.
The first DVDs (produced in 1997) had the widescreen version of the movie on one side of the disc, while the pan & scan version was on the other side. Also, films longer than about 2 hours had to be turned over to watch the last part of the movie. Now the trend is in having either just the widescreen version on the disc, with the entire film on one side, or having the widescreen as well as the pan & scan versions on one side. This is accomplished through a technique called Reverse Spiral Dual Layer (RSDL). For the first two hours, the laser beam is reading data on the top layer from the center of the disc to the outer edge. Then the laser beam shifts to read the deeper layer, from the outer edge of the disc back to the center. Because the disc continues to spin in the same direction, the spiral of the data track must be reversed. All DVD players, including the first generation, will read these RSDL discs.
DVD may also be used to store high sampling rate, long word length digital sound. DVD-A (DVD Audio) is DVD with music only, and the discs have 5.1 sound at 24 bit - 96 kHz sampling, or two-channels at 24 bit - 192 kHz sampling. There are some Dolby Digital DVDs, but DVD-A really just refers to uncompressed music discs (Dolby Digital is compressed). Meridian Lossless Packing (MLP) allows more data to be stored on the DVDs. Otherwise, 5.1 DVD-A would have been limited to 24 bit - 48 kHz.
Sony's latest format is called SACD (Super Audio Compact Disc), using 1 bit technology (Direct Stream Digital - DSD). The first discs are two-channel stereo only, but down the road, it will have 5.1 channels too. SACD needs no filters in the signal path, unlike PCM systems, and the lack of these filters should make for a higher sound quality. Keep in mind that SACD is a type of DVD-A, as the other music-only DVDs discussed above. DVD-V would refer to all DVDs that have video on them, and DVD-A is the category for DVDs with just music.
My own dream for DVD audio would be to have 8 channels of sound (Front left/center/right, Side left/right, Rear left/center/right) sampled at 500 kHz (the limit, at which no further audible improvements can be detected with increased sampling rate) with 24 bit word length. This would require a bitstream of 100 Megabits (Mb) per second just for the music data, not the overhead management data. A current technology DVD, using both layers and both sides, would only hold a few minutes of music in this form, and DVD is limited to 10 Mb per second anyway, so I guess we will have to wait until DVDs will hold a terabyte (1,000 gigabytes) or so, along with the ability to handle much higher bit rates.
Video and Sound Tracks
Laserdiscs (LD) use much of the same technology as CDs. They come in three different sizes: 12 inch (movies), 8 inch (music video), and 5 inch (music video), although the 8 inch and 5 inch laserdiscs are pretty rare these days. Many people don't realize that a laserdisc player will handle compact discs as well as movie discs. Therefore, a laserdisc player can be a multiservice unit for the home theater and for music listening too. The 12 inch LD, which is used for movies, has five tracks: one for the video, and four tracks of sound. The video and two of the sound tracks are analog and combined in what is called a multiplexed signal, bound to a carrier frequency, much like a television signal that is broadcast over the air. These analog data are embedded in the carrier frequencies by Pulse Width Modulation (PWM), meaning that the pits are on the laserdisc are of different lengths, compared to digital PCM tracks where the pits representing 1s are of the same length. The other two sound tracks are digital sound that are multiplexed separately, similar to regular CD sound tracks (PCM). Currently, when you watch a movie on laserdisc, the stereo sound is carried on the digital sound tracks, while the two analog sound tracks multiplexed with the video are used for commentary by actors and the director of the movie, or simply a duplication of the sound from the digital tracks. Most of the wide screen movies that are available for rental or purchase, are on laserdiscs and DVDs, so if you are a movie buff, and want to watch them in their original aspect ratio, purchasing a laserdisc player or a DVD player is a must. Even though there are some movies available in wide screen VHS, the quality of the image is not nearly as good as it is on laserdisc and DVD. Also, digital surround sound such as AC-3 and DTS are only available on the discs.
If you are going to combine a music listening system with home theater, a laserdisc player or DVD player can suffice for playing movie discs and CDs. Laserdisc and DVD players automatically recognizes which type of disc has been placed into the machine, and send the proper signal to the output jacks. Thus, let us assume that you will be purchasing only the laserdisc player or DVD player and will be using it with all discs (movies and CD music). In general, they all produce a very nice image for movies, and provide reasonable quality CD sound. What distinguishes them most are features (see below). Of course, a dedicated CD player can be added, especially if you like to put several CDs in the player at one time (carousel), and dedicated CD players will generally have higher quality sound than what can be obtained by using the CD capability in a laserdisc or DVD player. The more expensive CD players often have a transport separate from the DAC. It is not necessary to buy the same brand of DAC as the transport, and if you go this route, you should listen to different combinations, because they will all have their own distinct sound personalities.
LD and DVD Features
CLV and CAV
There are a number of features on the laserdisc player or DVD that you should be on the lookout for. First, it should have the capacity to perform visual search forward and reverse (you see the image moving quickly forward or backward, to relocate an interesting scene), and to freeze single frames. This should be capable in both the CLV (Constant Linear Velocity) and CAV (Constant Angular Velocity) formats for LD players (DVD players only use one format). Constant Linear Velocity means that the laser beam is reading data along the spiral track at a constant speed (11 meters per second for laserdisc, 1.2 meters per second for compact disc). As the track spirals from the center of the laserdisc to the outside edge, the rotation speed has to slow down from 1800rpm to 600rpm (500rpm to 200rpm for compact disc), in order to maintain this constant linear velocity of reading the data (the circumference of the outer edge is greater than it is at the center). Constant Angular Velocity means that the angle beween a line drawn perpendicular to the radius and through the spot where the laser beam is reading data, and the curve of the spiral track, remains essentially constant. In practical terms, this just means that the CAV disc rotates at a constant speed all the time (1800rpm). LPs, then, would also be considered a type of CAV disc, rotating at a constant 33 1/3 rpm.
Compact Discs are fundamentally in the CLV format, and because there are only two digital sound tracks on them, compared to one video and four audio tracks for a laserdisc, a compact disc can fit about 1 hour of music on a 5 inch disc. Most laserdisc movies are produced in the CLV format, because it allows 1 hour of movie to be stored on each side of the 12 inch disc (unlike CDs, the LD has information on both sides). In the CAV format, only 30 minutes of movie per side can be stored on laserdiscs. All laserdisc players can perform the visual search and freeze frame functions on CAV laserdiscs. However, only the higher quality players can do these things on a CLV disc. All DVD players have the search function. Being able to do fast forward or reverse, and see the image while doing so, is very handy when you see something you want to watch over again, and especially if you want to view a special effect frame by frame. So, if these features are important to you, be sure to check that the laserdisc player you are considering for purchase can perform them on CLV discs.
Playing Both Sides
One other feature that is very convenient is the ability to play both sides without having to turn the disc over. Many of the movies released on laserdisc are less than two hours in length, which means that you can watch the entire film, if it is in CLV format, on one disc, without leaving your seat. As with televisions, a close look at the remote control unit is in order, for ease of use, and presence of buttons to activate the special features mentioned above. DVD players currently only play one side. You have to turn the disc over manually if the movie is longer than about two hours. This will depend on how much video compression, called MPEG-2, has been applied to the movie. DVDs have not only the two sides to the disc, but each side has two layers. All DVD players will play both layers (upper and lower) of each side of the disc (turning the disc over manually).
The back of the LD and DVD player will have at least one S-Video output, at least one pair of analog audio output jacks, and a digital output jack (either coax or optical). Late model LD players have an RF output which handles the AC-3 (Dolby Digital, DD) signal on laserdiscs encoded with DD sound. A Dolby Digital decoder is necessary to use this output, and an RF demodulator is also required. Some Dolby Digital decoders have RF demodulation on board with DD decoding. Others require a separate RF demodulator.
The analog audio outputs of the players can be connected directly to the television (audio input jacks), along with the S-Video output. However, this will not deliver surround sound, but rather, only stereo. Preferably, the two analog outputs of the LD player should be connected to the input jacks of a receiver or processor for the amplification system (discussed in Section 5 of this primer), and the RF output of the LD player, or digital coax ouput of the DVD player should be connected to the input of a receiver or decoder for DD. The digital output jack of an LD player can be connected to a DTS decoder, which is also for 5.1 digital surround sound. If the LD player has a better comb filter (see Section 1 on TVs) than the TV, then the S-Video connection should be used. If the TV has the better comb filter, then use a composite video connection. In the case of DVDs, a comb filter is not necessary, so always use the S-Video connection.
The latest DVD players have component video outputs, for use with TVs that have component video inputs. Component video has better color than S-Video. Some of the newest receivers have component video switching. Although you can copy movies from laserdiscs onto tape, the quality of the copy is poor compared to the laserdisc if you use VHS, and still not as good even if you use S-VHS. DVDs cannot be copied onto tape, as they have sophisticated copy protection.
Dolby Digital (DD) and DTS
Dolby Digital (DD) is available on the later laserdiscs (AC-3 is the codec, and Dolby Digital is the marketing term) and all DVDs. Laserdisc players equipped for DD have an RF output jack that is connected either directly to receivers that have an AC-3 RF input, or to an outboard RF demodulator that is then connected to the DD digital input on the receiver. Dolby Digital is referred to as a 5.1 channel system, which means that there are 5 discrete (separate) channels of surround sound and 1 channel for a subwoofer or possibly special effects. The audio bits from all 6 channels (5 + the .1) are carried in the right analog channel of laserdiscs, using a technique called data reduction (compression). The left analog channel contains a mono version of the movie sound track or audio commentary by the director. Older laserdisc players can be adapted for use with Dolby Digital, but the player must be sent to specialty shops for this modification, and it costs about $400. DIY mods for this cost about $75.
There is another 5.1 digital surround format competing with DD, called DTS, and which is available on numerous laserdiscs, DVDs, as well as CDs. DTS occupies the two digital tracks on the laserdisc rather than the right analog track as is the case for Dolby Digital (DD). DTS uses less compression (called "lossy" as opposed to a "lossless" audio format) than DD, but DD technology encodes only those sounds that the ears (brain) pay attention to, governed by psychoacoustic principles. This is called "Perceptual Coding". The algorithms (mathematical formulas) that govern the coding and decoding process of DD and DTS are called "Codecs" (CodingDecoding). Encoded DD sound utilizes 384 kilobits per second for all 5.1 tracks, compared to about 1.5 megabits per second for both of the regular two track stereo CD sound. DTS uses somewhat higher bitrates than DD, but still not nearly as much as a regular CD. Both DD and DTS can have up to 24 bit word lengths but are often 20 bit or even just 16 bit depending on the amount of space the DVD producer wants to use. Even at 24 bit though, DD and DTS are both still compressed formats.
DVDs with DTS encoded sound are now available, both as DVD-A and DVD-V (movies). Although some of the earlier DVD players would not output the DTS bitstream, they all do now. They have a logo on the front that says "DTS Digital Out" to let you know they are DTS compatible.
If you have an HDTV, you should purchase a DVD player that has component video outputs and progressive video out, so that you can get the most from the DVDs. The progressive video will be part of the signal emerging from the component video jacks on the player. This will eliminate scanning lines and provide superb color quality.
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