With the invention of digital recording, a new piece of equipment was introduced to the professional audio world, the convertor. Found in just about every piece of personal digital audio equipment, the function of a converter is to change an electric signal into digital data and vice versa. There are two ways a converter can work: from analog to digital (A/D) and from digital to analog (D/A). A/D converters can be found on the recording side of your signal chain, and a D/A converter will be utilized on the monitoring side. D/A converters can also be used to send signals to outboard equipment for processing.
When a converter takes a sample of the analog signal, it is transposed into binary code. Binary code is a series of zeroes and ones (on and off) that the computer can interpret. The number of zeros and ones in a sample is called a bit rate. In CD quality wav files, the bit rate is 16. This means that in each sample, there are 16 combinations of zeros or ones. This bit rate translates to the dynamic resolution of the sound or how quiet the quiets are and how loud the louds are. No matter what the bit rate is when the sound becomes too loud or above 0db, it will “clip hard.” When this happens, the sound will cut in and out, similar to the sound of a blown speaker. Professional audio converters can range from 16 bit to 32-bit floating point bit rates. A 32-bit rate floating-point converter is actually a 24-bit rate converter, but when the sound gets too loud, instead of clipping at 0db, it engages the extra 8 bits of data. In this way, it acts like analog equipment extending your headroom.
The sample rate is how many times per second a sample of the sound is captured. In many ways, a convertor works like a video camera. The sample rate would be the equivalent of how many times per second a frame of film is captured. Common sample rates for converters are 44.1 kHz, which is CD quality, and 48 kHz for video or DVD projects. Other sample rates can include 88.2, 96, and 192 kHz. It can be argued that higher sample rates are not necessary for two reasons. One, Nyquist Therom states that for faithful reproduction of the audio, the sample rate must be at least twice the frequency source. Two, the final digital product will end up with a 44.1 or 88.2 KHz sample rate. Something essential to keep in mind when selecting your sample rate is as the sample rates get higher, the data files will get larger. Due to some of the limited space of hard drives, this can cause issues with storage and file management.
Another part of the convertor’s quality can be related to its clock source. The clock source is part of the convertor’s circuit, which is in charge of determining when the samples are taken. It can be internal or external to the unit. The physical source of this clock is a vibrating crystal, which resonates at a specific frequency. The crystal frequency vibrates in the megahertz allowing the converter to make precisely timed samples of the incoming signal. Just as important as the sample rate is the amount of jitter in the clock. Every clock exhibits some level of jitter. Jitter is when the spacing between samples, measured in nanoseconds, is not exact from sample to sample. The more jitter, the more offset the samples are. A high jitter converter will exhibit a “blurring” in the recording and playback. This is particularly significant to high frequencies.
When used in conjunction, convertors need to operate off of one clock source. Sync is a term used to describe how convertors work together when more than one is used collectively. Convertors must be synced where one is the master, and the others are its slaves. This will allow the different tracks on the convertors to be recorded and played back in synchronization. You can sync converters in series or parallel. Audio equipment connected in series is referred to as daisy chained.
An analog-to-digital converter cannot make instantaneous conversions. If it did, there would be an infinite number of samples. Instead, it opens and closes a logic gate allowing voltages to be stored on a capacitor and then processed. The logic gate works a lot like the shutter on a camera, opening and closing to allow light in or, in this case, sound. When the gate does not open and close on every sample, it causes some distortion which will appear in the signal. Most converters will apply dither to keep that gate open on every sample, even when there is no sound. Dither is a broadband noise, sometimes called white noise.
Many converters will also employ an anti aliasing filter. It is a low-pass filter that cuts out frequencies above half the sample rate. In the case of a 44.1 kHz sample rate, all the data above 22.05 kHz will be cut off. This is to satisfy the Nyquest Therom. It is important not to overdrive an analog-to-digital converter that utilizes an anti aliasing filter, as it can cause a failure in the filter. This filter can also help to eliminate any phase anomalies that could occur outside the human hearing range.
Some converters manufactures will advertise the quality of their products around their max sampling rates. This is not their most important characteristic. Some more important characteristics to the converters are its clock, noise floor and headroom. Overdriving a converter with no headroom becomes easy and noisy converters are always a bad thing. Some converters employ multiple clocks or crystal oscillators. In this type of design one is used to do the actual triggering of the logic gate and the other is used to make sure the triggering clock is staying in sync. When it comes to a converter’s quality it is not measured by its max bit rate or sampling rate but more on its overall topology characteristics.