I've just been reading Paul White's very useful article (SOS August 2000) replying to FAQs on digital audio. I am really a beginner in this field, having just bought my first CD‑R, a Tascam CDRW700. Unfortunately, I'm finding that I get different results depending on whether I record via Digital Direct mode or with it off. I have to record with it off if I need to also adjust the level of the digital‑to‑digital recording. Presumably, the extra circuitry used with Digital Direct mode 'off' is causing some extra errors?
Dr. Richard Fong
Assistant Editor Mike Senior replies: There's no extra circuitry involved in the CDRW700's digital gain control, as it operates in the digital domain, rather than the analogue. Digital gain change, at its most basic, involves each sample value being multiplied or divided. The product of each such calculation is usually not a value that can be represented accurately by a 16‑bit value, and hence the resulting waveform will not only be higher in level, but also noisier.
To illustrate this, let's take the single sample value of 31 and imagine that we want to increase its level by 50 percent: this would, in theory, give you a multiplication result of 45.5, which would have to either be represented as a sample value of 45 or 46, both of which would result in a small amount of error. This is probably one of the reasons why you're experiencing differences between the Tascam's different modes.
The CDRW700's manual isn't specific as to whether the results of digital gain adjustments are re‑dithered, as they should be, before being written to disk. If the results have not been re‑dithered, then any noise that has been introduced during the multiplication process will sound at its worst, rather than being masked out, and this may also be a factor in the sonic change.
There is possibly an additional reason for this qualitative difference in sound quality — the automatic sample‑rate converter with which this model of CD recorder is equipped. This is switched in whenever you leave Digital Direct mode and automatically resolves any other sample rate to 44.1kHz. Whatever digital source you're feeding the Tascam with is likely to have at least some variation or jitter in its clocking speeds compared to the Tascam (even if it is also nominally working at 44.1kHz) and so the sample‑rate converter might be automatically kicking in to try to remedy this, introducing inevitable low‑level sonic artefacts from the conversion calculations.
The best way around these problems for you is to only use Digital Direct Mode — if your recordings are digital already, then you are only going to make them sound worse by changing their gain digitally, even though you may even out some level differences, if you are doing it for the purposes of compilation. It would be much more advisable to record your original CD from analogue at a higher level in the first place, or to re‑assess the gain structure of your original mix if you did this in digital domain as well.