(To read this series from the beginning, start here.)
If you’ve followed this series from the start, you’ve seen our sound signal excite a microphone into generating an electrical signal, which has been amplified, converted to a digital signal, stored in a DAW and mixed with other recorded signals to build a mix, and even a master recording.
So how does this recording reach your ears, and the ears of your greatest fans? Well, in order for this recording to become sound in the air (even if it’s just the air between a set of earbuds and the closest pair of ear drums), the recording must first be converted back to an analogue voltage.
Now, unless you’ll actually be storing the music on an analogue system (vinyl, magnetic tape, or wax cylinders), this conversion from digital back to analogue will happen every time the recording is played back by a broadcaster or a listener. In the recording studio, we have control over the device which converts our amplified microphone’s signal into a digital signal, but it is likely that on playback we don’t have control over the choice of converter. This might take place in your recording studio, or it might take place inside a mobile phone to send the music to those earbuds I mentioned earlier. Really, all we can generally do is assume that our listener is hearing our music through a decent converter, but know that the quality of the converter will make a difference to the quality of the output, and to choose the best converters at our disposal when we do have control over the playback system.
For the purpose of understanding the terminology, these converters are Digital-to-Analogue converters, called D/A converters for short.
Some Guesswork Involved
Remember back when we talked about sampling the analogue voltages in order to generate a digital approximation? Well, in D/A conversion, we’re going from a sampled digital approximation back to a continuously-flowing electrical voltage.
The digital signal is stepped from one sample to the next, but as long as we chose a suitably high sample rate and bit depth at the time of A/D conversion, we can expect to not notice these steps in the reconstructed analogue signal. Between these steps, from one sample to the next, the D/A converter uses a practice called “interpolation”, which is essentially a guess of the actual shape of the voltage curve between the steps, based on the samples which come before and after. This guesswork introduces a certain amount of error in properly reproducing the analogue signal, but once again, this can be minimized by choosing suitable high sample rate and bit depth for the digital audio files.
Let’s Have a Peak
One ramification of the interpolation (guesswork) at recreating the shape of the analogue voltage curve, is that we can find the D/A converter generating a voltage shape which rises higher than the highest (loudest) samples in the digital audio signal. Imagine a series of samples which represent a rising curve followed by two highest samples and then a downward curve. In order to interpolate a smooth, continuous rising and falling curve from these samples, the D/A converter will likely set the peak of the curve between the two highest samples, at a voltage higher than that represented by either of the two samples themselves. This is called the “inter-sample peak”.
This situation is likely the best guess at the actual continuous flowing curve represented by the many stepped digital samples. To generate an analogue signal any other way – for example, a flat line (constant voltage) between the two highest samples, would be to knowingly introduce an error (distortion) into the analogue signal. The danger, here – if the digital audio file (the master) is prepared in such a way that the highest samples approach the volume limit that the bit depth can represent, this higher peak generated by the D/A converter may end up being louder than allowable by the analogue circuits, resulting in distortion of the analogue signal. The take-away message here – when preparing the master audio, leave a little “headroom” above the highest digital samples to allow the D/A converter room to maneuver.
Say No More
For the purpose of this practical discussion, that’s really all there is to be said about D/A converters and how to feed them properly. Choose the best converters at your disposal, and leave them a little headroom in the digital signal so that they can operate at their best.
And if you can’t find your converter, that’s because most converters are contained within your computer’s audio card/interface, or the digital device (phone, tablet, TV tuner) you’re using to select music for playback.
In the next instalment, it’s time to talk about amplifying the analogue signal and feeding it to your speakers.
Next Instalment: Part Nine – Speak Up!