Know Audio: It All Relies upon On The DAC

Our journey by means of the world of audio know-how has taken us step-by step out of your ears right into a typical residence Hello-Fi system. We’ve seen the audio system and the amplifier, now it’s time to try what feeds that amplifier.

Right here, we encounter the primary digital part in our journey outwards from the ear, the Digital to Analogue Converter, or DAC. This circuit, which you’ll discover as an built-in circuit, takes the digital data and turns it into the analogue voltage required by the amplifier.

There are numerous requirements for digital audio, however on this context that utilized by the CD is most typical. CDs pattern audio at 44.1 kHz 16 bit, which is to say they categorical the extent as a 16-bit quantity 44100 occasions per second for every of the stereo channels. There’s {an electrical} customary referred to as i2s for speaking this knowledge, consisting of a serial knowledge line, a clock line, and an LRclock line that signifies whether or not the present knowledge is for the left or the fitting channel. We lined i2s intimately again in 2019, and do you have to peer into nearly any shopper digital audio product you’ll discover it someplace.

Making A DAC Is Simple. Making A Good DAC, Not So A lot.

The Philips I2C to parallel converter.The Philips i2s to parallel converter.

Remembering that i2s is a know-how from the tip of the Nineteen Seventies, it’s a surprisingly easy one to create a DAC for. The unique Philips specification doc accommodates a circuit utilizing shift registers and latches to seize the samples, which may be fed to a easy resistor ladder and filter to carry out the conversion. That is an efficient solution to flip digital to analogue, however as with each audio part it carries with it a degree of distortion.

In case you take a look at the output of any DAC within the frequency area quite than the time area, you’ll discover noise together with the spectrum of no matter sign it’s processing. For example, the sampling frequency can be there, as will a mess of spurious mixer merchandise derived from it and the sign. In an audio DAC all this out-of-band noise can present itself as distortion.

The issue confronted by audio DAC designers is that the sampling frequency is comparatively near the sign frequency, so whereas the low-pass filter does its greatest to take away the offending spectrum, it has a tough job. For instance again in 2020 we took a take a look at the CampZone 2020 badge, an audio playground that used a really low cost DAC to maintain prices down. The Shenzhen Titan TM8211 i2s DAC is a single-chip implementation of one thing near that Philips DAC circuit, and whereas it boasts an impressively low value, the noise is clearly audible on its output in a method that it wouldn’t be on a dearer chip.

Shifting The Downside Upwards For A Higher Sound

A pair of Philips TDA1541 chips on a circuit boardThe Philips TDA1541 was the archetypal oversampling DAC of the late Nineteen Eighties and Nineties. Cjp24, CC BY-SA 3.0 .

The answer discovered by the DAC designers of the Nineteen Eighties and Nineties was to maneuver that out-of-band noise up in frequency such that it may very well be extra effectively rejected by the filter. In case you bear in mind CD gamers years in the past boasting “oversampling”, “Bitstream”, or “1-bit DAC”, these referred to the event of extra superior DAC designs that carried out the shift upwards in out-of-band noise frequency by numerous totally different methods. On the time this was a hotly contested advertising and marketing warfare between producers as a CD participant was seen as a premium machine, from a place three many years later when a CD is one thing that must be defined to youngsters who’ve by no means seen one.

All of those are primarily sigma-delta DACs, and so they method the issue of transferring the out-of-band noise upwards by producing pulse chains at a excessive a number of of the pattern clock the place the variety of pulses corresponds to the worth of the pattern being transformed. By sampling with decrease decision, however a lot sooner, the related out of band noise is shifted a lot larger up the frequency vary, which makes the job of separating it out from the sign a lot simpler. It may be decoded into an analogue sign via a reasonably simple low-pass filter. These are the “Bitstream” and “1-bit” DACs marketed on these Nineties CD gamers, and what was as soon as the bleeding fringe of audio know-how is now commonplace.

A Good DAC Can’t Compensate For A Poor Supply

Though a superb DAC makes an enormous contribution to audio high quality, we’ve assumed that the digital knowledge comes from a supply with out a lot compression, resembling a CD. CDs are actually now not mainstream, and the information is more likely to have come from a compressed supply resembling an MP3 file or an audio streaming service. Compression is a subject in itself, however it’s price making the purpose that the standard of the audio expressed on the information stream displays the traits of the compression algorithm used, and irrespective of how good the DAC could also be it cannot make up for the standard of its supply.

This collection will return for its subsequent installment, the place we’ll handle the issues of the vinyl and tape aficionados who had been screaming on the assumption that there can be a DAC within the sign chain. Whereas once-dominant analogue audio codecs such because the LP report and the cassette tape might now command a fraction of the market they as soon as had, their rediscovery in recent times has led to a minor resurgence of their reputation. It’s nonetheless under no circumstances uncommon for a high-end audio system to have analogue supply parts, so they’re very a lot price a glance.