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Inner workings

— By Paul McGowan —
We're starting down the path of trying to unravel a bit of the topic why the analog output stage of a DAC is so important: perhaps more important than the DAC itself. To begin let's first take a DAC apart into its 4 main categories so we can start with a clear understanding of what's inside these devices. The purpose of a Digital to Analog Converter is just what the name implies: converting digital audio to analog audio. We've learned in past posts that one type of digital audio, known as DSD, can be used directly to feed an analog power amplifier without decoding (although no one does this because it's quite noisy without further processing). When we refer to digital audio, in this series, we will be speaking about PCM which cannot be listened to directly as the code is based on numeric representations of the audio. DACS have 4 main components:
  • Input receiver
  • The DAC itself (including an upsampler, digital filter, ladder or DSD engine)
  • Current to voltage converter
  • Output analog stage
Today we'll cover the input receiver in a bit of detail. The goal of the input receiver is to convert any incoming data to I2S, the native format for all PCM DACS. I2S has 4 components, 3 of which are just clocks, the 4th is the actual musical data:
  • Bit clock
  • Word clock
  • Master clock
  • Data
PS Audio DACS, as well as a growing number of other manufacturer's DACS, have adopted the PS open standard for I2S over HDMI. This method eliminates the need for the input receiver, so we'll ignore this for right now. The vast majority of all modern DACS get their data through a single stream called S/PDIF (Sony Philips Digital Interface), USB or Ethernet/WIFI . In any of these cases we need to break apart the data and wind up with our 3 clocks and data. Since the scope of this series is limited, let's just focus on what most people have coming into their DACS: S/PDIF from a CD player or a computer sound card. When a CD player or computer sound card gets the information off of a disc (optical or hard) that information is sent using the same I2S components that we need to get into the DAC. However, Sony and Philips decided (in their non-Audiophile friendly) collective wisdom that it would be inconvenient to have a 4-conductor cable to send I2S to an external DAC. Instead, thought they, it would be simpler if they just smooshed all the clocks and data together so it could be sent over a single cable and then decoded at the other end. Certainly there are positives and negatives with this approach, but it is what we have to live with. So the job of the input receiver on a DAC is to disassemble the smooshed clocks and data as best it can and send that data to the DAC. In the process of recovering the clocks and the data we have our first signs of trouble - which years ago we solved with an accessory product called the Digital Lens. Today, however, many DACS have well designed receivers that, while not perfect, get the job done with good results. USB, popular today, is a little friendlier if the DAC has an asynchronous input receiver because the master clock is not included in the data and is generated by the receiver itself. So now we have our data in the form we wish, 3 clocks and 1 data stream. Tomorrow we'll convert the numbers into audio with the 2nd part of the DAC, the DAC itself.

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