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Q. Can I feed a 24-bit signal into a 16-bit device?

By Hugh Robjohns & Tom Flint

The Akai DPS12 multitrack recorder has a fixed 16-bit S/PDIF input.The Akai DPS12 multitrack recorder has a fixed 16-bit S/PDIF input.

I want to record eight tracks at once into my Akai DPS12 multitracker, which has six analogue inputs and a stereo S/PDIF input. All the sources will be analogue, either from mics or DI boxes. As I need to get a portable mixer/preamp box for this project as well, I thought a Soundcraft M-series mixer would be perfect, as it has an S/PDIF output in addition to direct outs and would easily allow me to put down the required number of tracks. However, the S/PDIF output is fixed at 24-bit on the Soundcraft, and the Akai's S/PDIF input is fixed at 16-bit. The digital outputs on the other options I've looked at, such as the Focusrite Octopre, are also fixed at 24-bit, which seems to be the standard these days. So now I'm looking at getting a completely analogue mixer and a separate A-D converter to convert two of the outputs. Can you see another solution?

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Technical Editor Hugh Robjohns replies: Feeding a 24-bit signal to a 16-bit device will result in the truncation of the bottom eight bits — the device receiving the signal simply ignores the bottom eight bits. This effectively raises the noise floor by more than 40dB but, more importantly, will increase the amount of distortion considerably, especially for low-level signals. This will tend to make things sound 'hard' and 'grainy', but just how noticeable and/or objectionable this will be depends on the types of music you're recording and the level of quality you expect.

Ideally, you should find a way of re-dithering the 24-bit signal to 16-bit before outputting it from one machine to the other. Pretty much all fully digital sound desks and computer DAWs have facilities for this, but few modern A-D converters or digital signal processing devices have the capability — most operate with a fixed 24-bit resolution, as you say.

It's worth noting that the S/PDIF output on the Soundcraft M-series mixers will only carry the main stereo output signal, so if you want to use it to get two separate channels of audio into the Akai, you won't be able to use the mixer for anything else at the same time.

As an alternative to the Soundcraft M-series mixer, you could look out for a second-hand digital desk, like a Soundcraft 328 perhaps, or a Yamaha 01V. All modern digital desks will offer switchable output resolution. If you decide to go with an analogue mixer and buy a separate stereo A-D converter, find one that offers switchable output resolution, so that when you upgrade the recorder you won't have to upgrade the A-D as well.

SOS contributor Tom Flint adds: A few years ago, when an Akai DPS16 was my main recording device, I encountered the same dilemma. In my case, I wanted to record 10 tracks simultaneously using the DPS16's eight analogue inputs plus its stereo S/PDIF in. I first began looking for a stand-alone A-D converter which would allow me to feed the Akai with the stereo analogue output from one of my sound modules, but most dedicated converters were very pricey. Then I noticed that the Lexicon MPX100 multi-effects processor had an S/PDIF output and a Bypass button, which made it possible to use it to convert any stereo analogue input into digital, with or without using the internal effects. I picked on the MPX100 purely because it offered a reasonable specification at a very low retail price. I wasn't actually in the market for an effects box, but having some Lexicon algorithms available was a bonus.

In terms of compatibility, the MPX100's S/PDIF output is 20-bit, which matches neither the 16- nor 24-bit modes offered by the Akai DPS16. Ideally, some sort of dithering was required, but my intention was to feed the Lexicon with the signal from a drum machine or sound module, and not for anything really sensitive, like vocals. I managed to borrow an MPX100 to test how much difference the lack of dither would make, and found it not to be a problem, so I went ahead and made the purchase. I certainly don't think it would be worth spending large sums of money on additional equipment just to get around the bit-rate discrepancy, particularly for the sake of the DPS12, which is a budget product, and could easily be replaced with a machine that records up to eight tracks or more.

Matching the bit rate wasn't the only consideration, though. In order to use the digital input, the Akai had to be slaved to the incoming digital signal, and therefore be locked to the Lexicon's internal clock rather than its own. This was a cause for concern, because it meant that the Akai's A-D performance could be compromised by a poor pulse, just as it could be improved if fed a more stable clock. I don't remember if I made any investigation into the relative quality of the clocks, but my own recording tests didn't lead me to think that the MPX was doing a poor clocking job in any way.

Sample rate was also a consideration because although the MPX has a fixed rate of 44.1kHz, the DPS16 can operate in 32, 44.1, 48 and 96kHz modes. Nevertheless, I eventually decided that I'd rather save disk space by running the Akai at 44.1 than try to obtain higher quality from a mid-priced machine that probably isn't quite good enough to demonstrate the benefits of 96kHz anyway.

Published February 2005