Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part I


Number of DACs per Chip

The number of DACs per chip is typically two or eight. The eight-channel chips are often called octal DACs. The ability to increase the number of channels on a single chip results from transistor scaling as process technology advances, at least for the digital transistors. Analog components often do not scale, since this can affect noise, distortion, and level matching between channels. It thus should come as no surprise that the parts with a lower performing analog section were the first to be made available as octal DACs. These parts also minimize the amount of silicon taken by the digital filter. The datasheets for lower priced octal chips show the performance degradation of the digital filters in which the most significant is the amount of attenuation in the stop band. Digital filter specifications will be discussed in Part II.

The performance of DACs with eight channels improved when Cirrus offered parts with worst case SNR and THD improved by 10dB over previous octal parts in 2001. In the last few years ESS has shown that the analog performance need not be compromised offering almost all its parts with eight independent channels.

The advantage of octal chips is that only one chip is required instead of four on the PC board. An added benefit is the matching between channels is improved since all 8 are on a single chip.

In highly integrated chips, the two ADCs for conversion of analog stereo inputs to digital for DSP processing may also be integrated on the same chip. When both ADCs and DACs are on the chip, it is called a CODEC.

The most highly integrated mixed signal ICs for AVRs may also include the SPDIF input selector, as well as SPDIF timing and data recovery system. This is sometimes called the DIR for Digital Input Receiver. Combined with the latest multicore DSP chip and a highly integrated analog chip, to be discussed below, most of the audio portion of an AVR are found on three chips.

Recovery of digital audio from the HDMI inputs is performed by ICs on the video switching board of the AVR. The PCM streams from USB and Apple digital audio dock cables are produced by other chips in the AVR.