Our journey through the world of audio technology has taken us step by step from your ears to a typical home Hi-Fi system. We have seen the speakers and the amplifier, now it’s time to take a look at what powers that amplifier.
Here we meet the first digital component on our outward journey of the ear, the digital-to-analog converter, or DAC. This circuit, which you will find as an integrated circuit, takes digital information and transforms it into the analog voltage required by the amplifier.
There are many standards for digital audio, but in this context, the one used by CD is the most common. CDs sample 44.1 kHz 16-bit audio, that is, they express the level as a 16-bit number 44,100 times per second for each of the stereo channels. There is an electrical standard called i2s for communicating this data, which consists of a serial data line, a clock line, and an LRclock line that indicates whether the current data is intended for the left or right channel. We covered the i2s in detail in 2019, and if you look into almost any consumer digital audio product, you’ll find them somewhere.
Making a DAC is easy. Make a good DAC, not so much.
Remembering that i2s is a technology from the late 1970s, it is a surprisingly simple technology to create a DAC. Philips’ original specification document contains a circuit using shift registers and latches to capture samples, which can be routed to a simple resistance ladder and filter to perform the conversion. It’s an efficient way to switch from digital to analog, but like any audio component, it comes with a level of distortion.
If you look at the output of any DAC in the frequency domain rather than the time domain, you will find noise as well as the spectrum of the signal it is processing. For example, the sample rate will be present, as will a multitude of parasitic mixing products derived from it and the signal. In an audio DAC, all of this out-of-band noise can manifest itself as distortion.
The problem encountered by audio DAC designers is that the sample rate is relatively close to the signal rate. So even though the low pass filter does its best to remove the offending spectrum, it has a tough job. As an example in 2020, we looked at the CampZone 2020 badge, an audio playground that used a very cheap DAC to keep costs down. The Shenzhen Titan TM8211 i2s DAC is a single-chip implementation of something close to this Philips DAC circuit, and although it enjoys an incredibly low price, noise is clearly audible on its output in a way that it wouldn’t be on a more expensive chip .
Move the problem up for better sound
The solution found by the DAC designers of the 1980s and 1990s was to shift this out-of-band noise up in frequency so that it could be rejected more efficiently by the filter. If you remember the CD players of years ago that offered “oversampling”, “bitstream” or “1-bit DAC”, they referred to the development of more advanced DAC designs that shifted upward. out-of-band noise frequency by different technique. At the time, it was a hotly contested marketing war between manufacturers, as a CD player was considered a high-end device, from a position three decades later where a CD was somehow. something that must be explained to children who have never seen one.
All of these DACs are essentially sigma-delta DACs, and they address the problem of moving out-of-band noise upward by producing chains of pulses at a high multiple of the sample clock where the number of pulses matches. to the value of the current sample. converted. By sampling at a lower, but much faster resolution, the associated out-of-band noise is moved much higher in the frequency range, making it much easier to separate it from the signal. It can be decoded into an analog signal by means of a fairly simple low pass filter. These are the “Bitstream” and “1-bit” DACs advertised on those CD players from the 1990s, and what was once the cutting edge of audio technology is now commonplace.
A good DAC cannot compensate for a bad source
While a good DAC is a huge contributor to audio quality, we’ve assumed that digital data comes from a source without too much compression, like a CD. CDs are no longer common and data is much more likely to come from a compressed source such as an MP3 file or audio streaming service. Compression is a subject in itself, but it should be noted that the quality of the audio expressed over the data stream reflects the characteristics of the compression algorithm used, and no matter how good the DAC is, it cannot compensate for the quality of its source.
This series will return for its next installment, where we will address the concerns of vinyl and cassette enthusiasts who were crying out for the hypothesis that there would be a DAC in the signal chain. While the once dominant analog audio formats such as LP record and cassette may now represent a fraction of the market they once had, their rediscovery in recent years has led to a slight resurgence in their popularity. It’s still not uncommon for a high-end audio system to have analog source components, so they’re worth checking out.