Sound card recording

December 2, 2016
Original ICON Mobile U

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.

Headroom Headaches

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

Hardware Compressor Or Plug-in?
Some voice channels include analogue compressor stages, which can be very helpful in managing recording levels. A compressor can reduce the dynamic range of a signal and will therefore, if peak output levels are kept the same using its make-up gain control, effectively boost low-level signals. However, it's important not to overcompress when recording, as this can't be undone later - use a gentle compression with a ratio of less than 3:1 and then use software plug-ins to do any further dynamics processing you might require. Add too much compression and you're stuck with it, so try not to apply more than 6 to 8dB of gain reduction at this stage.

You may reasonably ask why, since compressors are available as plug-ins, we should use voice channels with analogue compressors at all - why not do all the compression using plug-ins? The answer is that you can do this, but that there are gain-structure factors to consider. The analogue compressor affects the signal before it hits the input converter, while a compressor plug-in comes after the converter. By using the analogue compressor pre-conversion, the dynamic range of the signal can be squeezed so that more bits are being used for more of the time. This in turn leads to less noise and distortion when in the digital domain. In practice, though, if you record an uncompressed signal paying proper attention to levels, adding moderate compression in the digital domain shouldn't cause any problems.

sound becomes progressively more distorted. In pop music, signals are invariably pushed several dBs 'into the red' to make use of the artificial warmth this can add.
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