Secrets Q & A

AVR DACs vs. Standalone?

Question:
Do the DACs (digital-to-analog converters) in mid-priced AV receivers perform as well as standalone DACs such as the Peachtree Audio DAC-iT or Musical Fidelity V-DAC II?

- Ed Wood
West Chester, PA

Answer:

Many AVRs, even those costing as little as $500 or so, have surprisingly good DACs these days, perhaps even on par with some standalone DACs. This is difficult to determine, since manufacturers rarely reveal the specific circuit components they use, but SECRETS senior editor David Rich has made an extensive study of AVR schematics and found that most use fairly high-performance DACs.

As with all audio products, there are several performance characteristics to consider with DACs, including frequency response, signal-to-noise ratio (S/N), and total harmonic distortion (THD). These characteristics are specified for AVRs overall, but not for the internal DAC individually.

Frequency response is the range of frequencies the device can reproduce within a certain tolerance. For example, the V-DAC II specifies a frequency response of 20 Hz to 20 kHz (+0, -0.1 dB), which is ruler-flat; Peachtree does not specify the frequency response of the DAC-iT.

S/N defines the maximum difference between the power of the intended signal and the power of the device's inherent noise; the higher this value is, the better. The DAC-iT specifies a S/N of 120 dB, which is very good; Musical Fidelity does not specify the S/N of the V-DAC II.

THD is a measure of how much a device changes, or distorts, the harmonics in the input signal; the lower the value, the better. The V-DAC II specifies a THD of 0.004% from 20 Hz to 20 kHz, which is excellent; Peachtree does not specify the THD of the DAC-iT.

Then there are characteristics related to the digital side of the DAC, but to understand them, you need to first know the basics of digital audio. (There are several forms of digital audio, but I'm going to limit this discussion to the most common one—PCM or pulse-code modulation.)

Sound is an inherently analog experience—quickly oscillating changes in air pressure, called sound waves, reach your ear and cause the eardrum to vibrate, which results in the sensation of hearing. To convert sound waves to digital information, they are first converted to an analog electrical signal by a microphone.

Then, an analog-to-digital converter (ADC) takes an instantaneous measurement of the changing voltage in that electrical signal many times per second, and each measurement is represented by a fixed number of digital bits. The rate at which the measurements are taken is called the sample rate, and the number of bits used to represent each measurement, or sample, is called the resolution or bit depth.

CDs use a sample rate of 44,100 measurements per second, which is expressed as 44.1 kHz, and a resolution of 16 bits; you'll sometimes see this written 44.1/16. By contrast, so-called high-resolution digital audio often uses a sample rate of 96 kHz and a resolution of 24 bits, or 96/24.

The sample rate determines the highest frequency that can be represented by the digital system—roughly half the sample rate. So for a sample rate of 44.1 kHz, the highest frequency that can be accurately represented is around 22 kHz.

The resolution determines the S/N of the audio; each bit added to the resolution theoretically increases the S/N by 6 dB. A resolution of 16 bits can represent a S/N of 96 dB, while 24 bits can represent a S/N of 144 dB, though few if any digital-audio devices can actually achieve these theoretical values.

A digital-audio bitstream must be converted back to analog in order to hear it, which is where the DAC comes in. Each sample is converted into an analog voltage, forming a stairstep waveform as shown above. Because such a waveform has lots of high harmonics not present in the original, the signal passes through a lowpass filter, which removes the high harmonics and smoothes out the stairsteps.

The timing of each sample as it is created in an ADC and converted back to an analog voltage in the DAC is controlled by a clock within the device. The ideal clock "ticks" at a perfectly constant rate, but real-world clocks are not that perfect—the timing between samples is not exactly constant. The error in this timing is called jitter, which can introduce noise and harmonic distortion into the final signal. Also, jitter can reduce the effective resolution of the signal—for example, in some cases, less than a nanosecond (billionth of a second) of jitter can reduce the resolution of a 44.1/16 bitstream to 14 bits, increasing S/N by 12 dB.

As you might imagine, the lower the jitter, the better. Musical Fidelity rates the jitter of the V-DAC II at less than 12 picoseconds (ps, trillionth of a second), while Peachtree specifies the DAC-iT's jitter at less than 3 ps. Both of these specs are very good, especially considering that 1 picosecond = 1/1000 nanosecond.

Each DAC has its own sonic character determined by the design, components, etc. The best way to find the one you like best is to listen to several if possible. Otherwise, read reviews from trusted sources. SECRETS has not reviewed the DAC-iT or V-DAC II, but we have reviewed others here.