Paul White offers a few tips on setting recording levels for soundcard-based music systems.
Everywhere we see glossy ads for pieces of kit that promise to make our recordings sound better, but the harsh reality is that most of us don't make the best use of what we already own. From a technical standpoint, good recording quality relies on paying attention to gain structure, and that's just as important when using computer-based soundcard studios as it was when everything was analogue.
Analogue To Digital Conversion
Even in an all-digital system, the source signal is almost always analogue, and for most of us that means that either a mixer channel or a voice-channel processor is needed, either to provide A-D conversion or to make sure that analogue signals feed your soundcard's own A-D converters at an optimum level. If you're going to get the best results from your setup then it's here that the process of gain management should start - you need to get the right signal arriving at your converters.
If you set the analogue input level to the converters too low, you lose resolution, while if you set it too high, the signal will clip. Resolution refers to the number of discrete steps that make up a digital signal and, as we all know, the more steps there are the more accurate the sampling process. If a signal comes in that is too low in level, it will cover fewer of these steps, and the digital representation of the analogue waveform will be less accurate than if it had used up all the bits. Figure 1 illustrates this point.
Confusingly, some soundcards include digital input-gain controls, but these must, by definition, act after the signal has been digitised and hence will not recover any lost resolution - once resolution is lost, boosting the signal level in the digital domain will also increase any background noise and distortion in the recorded signal.
Whereas the sound-quality of an analogue recording depends on the signal level you can get onto tape, digital recording quality is directly related to the signal level you can feed into the A-D converters. If your system does have a digital gain control, the best strategy is to set it to unity gain, then adjust the analogue source level until the correct meter reading is produced.
A number of soundcard users have noticed that they have to feed a very high analogue signal level into their card's line inputs in order to achieve a sensible reading on the digital meters. This is a deliberate design feature which aims to cope with the mechanics of analogue recording. Analogue tape doesn't have a well-defined maximum recording level beyond which you shouldn't record. It has a nominal maximum level above which the