What is a DAC?

DAC is an acronym for Digital-to-Analog Converter. A DAC transforms digital signal into analog signal that can be then used by an analog device. A common example would be to convert digital music to analog for an amplifier (inside headphones or speakers). DACs are used in a wide variety of applications including modems.

What’s an analog signal?

The sounds/music that we hear and record are transmitted by sound wave that propagate through the air as a continuous analog signal. It looks like a 2D waveform if we were to visualize it.

To record the analog signal into a digital form, the recording process measures the sound waves and saves it as binary information tens of thousands of times per second. For example, a 44 kHz audio file has been sampled (measured) 44,100 times per second.

The higher the sampling and the precision of each sample (16-bit, 24-bit, 32-bit or more), and the more accurate the digital recording is. When it’s time to play it back, the digital signal goes through a DAC to be converted back to analog form. The analog signal is then sent to a speaker amp to attempt to reproduce the original sound wave as accurately as possible.

The same fundamentals are true for pretty much all signal, including radio waves etc. which may not be audio/music related.

DAC challenges

It’s easy to think that converting from Digital to Analog is simple, but it is not. There are many challenges linked to DACs, and they mostly related to noise. Any electric/electronic component or circuit can generate unwanted noise, which means that unwanted noise is introduced in the original source source signal during the digital-to-analog conversion.

The easy way to explain audio noise is to compare it with image noise. Images are a color/visual signal that can also be subject to noise. If you look at this side by side image, it’s immediately clear why reducing noise in a visual signal makes a difference. The same is true for audio.

Since there is always “some” level of noise introduced by electronic components, it’s important to see how invasive that noise is in relation to the original signal. The metric for this is called signal-to-noise ratio or SNR or S/N expressed in decibels (dB). The higher this value, and the less the noise is noticeable.

Audio DAC

In the context of this site, we will most likely be talking about Audio DAC devices. With the introduction of smartphones like the LG V20 and its Quad-DAC in October 2016, it is likely that more OEMs will introduce similar capabilities to compete in audio playback.

DACs are present in a huge number of devices, going from very cheap ones, to very expensive ones. Any USB/Bluetooth speaker has a DAC, and high-end HiFi devices have DACs too. As you can guess, their quality and prices will differ greatly.

Not all DACs are created equal

Although their primary function is fair simple (convert digital signal to analog), DAC devices or chips from different vendors use very different, and sometimes extremely complex, strategies to get the job done. Some DACs are very simple and cheap, while others use extremely convoluted electronics, noise isolation and noise reduction techniques to provide a signal which is as good as possible.

Phones like the LG V20 uses the ESS 9218 chip which is a high-end DAC system from ESS Technology. It is a complete system on a chip (SoC) that contains four DACs which all work to produce a high-quality conversion for a single signal. There are many different types of enhanced DAC, and this is only one of the better example at the time of publishing.

USB and External DACs (audio)

Before DACs were built into phones, they frequently took the form of external boxes that would replace an internal DAC (generally for a computer or audio playback device). Receiving a digital signal over USB or via an Optical connection, external DACs provide a much better digital-to-analog conversion than the built-in DAC.

The concept is the same as an internal DAC, except that external DACs offer a different connectivity option for a very large range of (existing) devices. Don’t be fooled by the size: single-chip solutions can perform just as well as boxes the size of a cigarette pack.

Audio file formats for high-quality sound/music

Obviously, to output the best possible signal, it is required to have recorded and stored the original signal as accurately as possible. How that happens is an entirely different topic, but it’s worth pointing out the general differences in audio file formats.

If you are buying commercial high fidelity (HiFi) digital content, we’ll assume that the recording studios have the best possible recording quality. In general, you need to look for a few things:

• bitrate (higher is better)
• sampling frequency (higher is better)
• compression technique (uncompressed is better)

The bit-rate (16/24/32 bits) represents the amplitude of the recorded information. The sampling frequency shows how close to analog the digital recording is. Compressed audio does induce some information loss, but the files are much smaller as uncompressed files.

FLAC (Free Lossless Audio Codec) is currently a popular file format for high-resolution lossless audio. DSD (direct stream digital) is also a popular format which is used for Super Audio CDs. I’ve heard many professionals prefer DSD, but in reality, FLAC is easier to find as downloads. Apple users have the option of using ALAC (Apple Lossless Audio Codec) for uncompressed audio.

Keep in mind that common formats such as MP3 and AAC are compressed, and caters to the low-end tiers of the audio market. If you have a good DAC and a decent pair of headphones $50+, the difference is noticeable.

Hubert Nguyen
Categories: Audio