Technical & Editorial

AVR - Audio Video Receiver - Build Quality: Part I


Multiple DACs Combined to Produce a Single Channel

Data sheets for a company's top of the line two channel DACs often provide specifications for the chip running as a mono device. In this mode, the output of the two DACs are summed together so that a single output is produced. When this topology is used, the noise floor is reduced. If the DAC supplier intends a part to be used in mono mode, the SNR improvement is provided on the datasheet for mono DAC performance.

An internal digital control bit in the DAC sets the DAC to accept a single mono PCM input stream.

The simplest DACs to place in mono mode are the current mode topologies. The outputs of the two channels are connected together. This connection is specifically shown in the Analog Devices in the Analog Devices application manual below and is also described in the ESS datasheets. TI appears to require a more complex setup discussed in Part II of this article.

Mono modes are found only in top-of-the line DACs in a vendors lineup because it is typically cheaper to move up the product line for stereo DACs rather than stay at a lower rung and use two DAC chips in mono to produce a stereo signal. The only time this does not apply is if the lower rung parts are being used in very large quantities across a large number of AVRs at a specific company. The company would get a significant discount given the large quantities they purchase. The next rung up the ladder part would be targeted only for a top of the line AVR, and the price of the DAC could be much higher since high quantity discounts would not apply to it.

Octal ESS DACs provide to option to be wired as stereo DACs (four current output lines shorted together) or mono DACs (eight current output lines shorted together). Thus three sets of specifications are provided on the data sheet (mono, stereo and eight-channel). With the choice increased to three, it is less clear which ESS DAC will yield the lowest cost solution for an AVR that requires eight or more channels of conversion. Some products use a given ESS chip for mono or stereo operation for the main channels (L and R) and another single chip with the same part number for the remaining six outputs in a 7.1 system.

Cirrus Logic does not have a DAC which is specified to operate in a mono mode.

It is possible to add circuitry so an AVR with 6 stereo DACs (to provide 12 outputs for 11.1 applications) can be switched to use the DACs in mono for SNR improvement, but only 6 channels are available. Some 9.1 channel AVRs switch only the front channels to mono allowing 7.1 operation to be preserved.