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Mixdown Formats

What Should I Mix Down To?
Published April 2002
By Paul White

There are now lots of different ways to make a stereo master recording of your final mix, so how do you work out which one is best for you?

Blank CD-R discs from TDK.Back in the early days of home studios, there was little choice when it came to recording your stereo master — the serious guys used a Revox quarter-inch tape machine running at 7.5 or 15ips (inches per second) while everyone else got by with domestic open-reel tape recorders or even cassette decks. Even today, those old Revox machines can give fantastic results, but now they compete with DAT, Minidisc, CD-R, CD-RW and hard drive as a repository for your precious mixes. Before you can decide which one is best for you, you need to know the pros and cons of each format.

Digital Audio Tape

Although specialised 24-bit models are produced, the commonly available DAT (Digital Audio Tape) recorders are designed to work at 44.1 and 48kHz, recording 16-bit uncompressed audio, which gives them the ability to record in the format that is required for commercial CD or audio CD-R production (44.1kHz, 16-bit). On the face of it, this might seem to be as good as you'll ever need for CD production, but that's not necessarily the case. Recording your final mix in a 16-bit format is fine if it is your final mix, but most of the time you'll need to do some post-mix mastering, even if it's only adjusting levels or adding a bit of EQ and compression. Every time you process digital audio, a small amount of resolution is lost due to scaling or rounding-up/down errors, so in an ideal world it's better to record at more than 16 bits, then dither down to 16 bits at the very end of the mastering process, just before preparing your CD master.

How much this loss of resolution matters depends to some extent on the dynamic range of your music. When digital data is subjected to a loss of resolution, the effects are felt most at very low levels where fewer bits are being used to represent the signal. It follows then that classical or acoustic music with a very wide dynamic range is most at risk, while pop music with its limited dynamic range is least likely to be affected in any audible way. Since DAT was introduced, I've mastered many albums using source material from DAT and I've never encountered problems, provided that the original recording used virtually all the available headroom of the DAT medium. If the material is under-recorded by 20dB, then it might be a different story. The only caveat is that fades should be done at the mastering stage, not when you mix down to DAT, otherwise subsequent processing could cause the tail end of the fade to sound a little grainy.

Tascam's DA45 DAT machine records 16-bit on standard DAT tapes or 24-bit in High Resolution (HR) mode at double speed.Tascam's DA45 DAT machine records 16-bit on standard DAT tapes or 24-bit in High Resolution (HR) mode at double speed.If you feel you need more mastering resolution, then choosing one of the specialist, 24-bit DAT recorders is an option, but bear in mind that there are a lot of other digital tape recorders which can be used for mastering as well, including multitrack machines such as Alesis ADATs or Tascam DA-series machines. The later generations of these machines can record at resolutions above 16-bit, making them ideally suited for mix recording, though you may need a suitable digital interface to make the most of their resolution if you're mixing from a digital source, such as a soundcard or digital mixing console. You also need to be sure your soundcard/mixer is able to support data transfer at the higher resolution, otherwise you might find your signal truncated to 16 bits anyway. Another advantage of using a multitrack recorder for mixing is that there are enough tracks to accommodate surround mixing (5.1 needs six tracks) and an eight-track machine neatly accommodates a surround mix plus a regular stereo mix.

The disadvantage of any digital tape format is that the machines need to be well maintained and aligned to avoid problems and, even in the best-run studios, machines sometimes eat tape! For this reason, keeping backups of anything even vaguely important is a must.

Analogue Tape

Analogue tape reels.Analogue tape is full of technical imperfections, but there's no denying that it has a 'sound'. For that reason alone, some engineers are willing to risk wow, flutter and less-than-immaculate noise performance in the quest for warmer-sounding masters. If you're thinking of using analogue tape, choose a machine that can run at 15ips at least, and clean the heads before every recording. Make some test recordings to find out how far you can push the recording levels before warmth gives way to audible distortion, and consider using one of the newer high-energy tapes for a better signal-to-noise performance (though this means having your machine realigned to match the new tape). Machine alignment needs checking using a test tape at regular intervals.

If you do mix to analogue tape, it's likely that you will still edit and master the recording digitally, so your digital system will need to be fitted with high-quality, 24-bit analogue converters if fidelity is to be maintained. A software denoising package can be used to reduce the effects of tape hiss without introducing noticeable side effects, providing the amount of noise is sensibly low.

The disadvantage of analogue tape is that it is now quite expensive and it has to be kept in a stable environment. Tape machines also require regular maintenance and alignment and, as most analogue machines are now getting on in years, this can be a serious consideration. Analogue tape is, however, one of the more durable storage mediums around if looked after (storing the tapes in your wardrobe in a dust-free container is usually fine), and, unlike digital media which either work perfectly or drop out, with no stage in between, analogue deterioration is progressive and therefore easier to detect.

Minidisc & Data Compression

Minidisc recorders use compression to store the audio data.Minidisc recorders use compression to store the audio data.Mixing to Minidisc is cheap (both in terms of the recorders and the media), which makes it very tempting to the home recordist, and the storage medium is also reasonably robust. The sound quality is surprisingly good, given that the data compression uses less than a quarter of the storage space of an equivalent-length CD, but most golden-eared types would argue that uncompressed audio still sounds better, especially for acoustic music. If you're working on a PC-based system with a budget soundcard and you're recording pop or dance music for private release, then Minidisc should be perfectly adequate — let your ears decide rather than the spec sheet. One thing to watch out for, though, is that, while CDs and CD-Rs can be cloned with no appreciable loss of quality, attempting to clone a Minidisc to another Minidisc will entail a further round of data compression, so you won't get a perfect clone.

If you decide Minidisc is OK for your needs, then the best results will be obtained using the digital input, as the converters on the cheaper machines are pretty average. On machines with optical S/PDIF digital inputs, you may need to buy an optical/coaxial converter.

Some low-end multitrackers use MP3 or some similar form of aggressive data compression to make audio recording onto SmartMedia memory cards a possibility. Some low-end multitrackers use MP3 or some similar form of aggressive data compression to make audio recording onto SmartMedia memory cards a possibility. Some low-end multitrackers use MP3 or some similar form of aggressive data compression to make audio recording onto SmartMedia memory cards a possibility. The sound quality from such systems depends to a great extent on the type of sound being recorded, and complex sounds with lots of frequency components tend to suffer the most. Used with care, these machines can produce surprisingly good results that are far cleaner than could ever be achieved with cassette tape, but this is due in part to the fact that any effects added after recording help to hide the artefacts of the data compression. Reverb is particularly useful, as it replaces some of the missing low-level detail. However, it is not a good idea to use these machines for capturing stereo mixes, even if they are designed to be able to do that, as the mix will suffer a further stage of data compression and the 'healing' benefits of adding reverb will be lost. It's probably best to master either to an uncompressed digital format or to mix directly into your computer editing system, if you use one.

24-bit/96kHz On A CD?! — The Alesis Masterlink

When it comes to mixing down, one interesting option is the Alesis Masterlink. This combines a hard disk recorder/editor with a CD-R burner and can be used both to record mixes and to edit them prior to burning a master CD. Some basic editing and processing are included and, in addition to making Red Book-compatible audio CDs from your recordings, the machine can also archive high resolution audio recordings up to 24-bit/96kHz using a proprietary data format.

Alesis Masterlink.

Masterlink has both balanced XLR and unbalanced phono analogue I/O as well as AES-EBU (XLR) and S/PDIF (phono coaxial) digital I/O supporting sample rates of 44.1kHz, 48kHz, 88.2kHz and 96kHz. Sample rates higher than 44.1kHz or any bit depths greater than 16 are converted down to 44.1/16 automatically as the CD is burnt using noise-shaped dither to maximise dynamic range.

Music can be mixed directly to the hard drive, then assembled in a playlist where the track starts and ends can be trimmed, appropriate gaps set up, fades created and so on. Should DSP processing be needed, EQ, compression, limiting and normalisation are available for off-line processing prior to burning a master disc or backing up files as CD-R data. The DSP functions can be set up differently for each track, track spacing and track level can be adjusted and fades can be applied. This little box impressed me when I reviewed it back in SOS April 2000 and, with the increased interest in high-resolution audio, it offers a viable alternative to software solutions.

Dedicated audio CD-writing software, such as Roxio's Jam, allows you to create Red Book-compatible CD-Rs for duplication.

CD-R & CD-RW

Stand-alone CD-R recorders are now quite affordable and the media almost cost less than the boxes they come in! The audio quality is identical to that of pressed audio CDs and most machines are provided with both analogue and digital inputs, often with integral sample rate conversion. However, you won't be able to record at a higher bit-depth and then dither down later, so in terms of quality, you're no better off than mixing to DAT.

Mixing to CD-R can be problematic, because, in the real world, you often need to go through a mix several times to get everything right (unless your entire mix is automated) and you can't erase a duff mix to a CD-R — but then, with CD-Rs now costing so little, you can probably afford to have several tries, anyway! Nevertheless CD-RW is perhaps a better bet, as it enables you to erase unwanted mix passes and hence reduce the risk of using the wrong mix when you come to compile your album. Erasing tracks can usually only be done from last to first, in order, and it can be fiddly to do, but at least it's possible.

If you choose a CD recorder that can burn Red Book compatible discs in disc-at-once mode, you may also be able to use it to record album masters from your computer after editing. However, to be realistic, most computer systems now come with CD-burning capability, and Red Book CD compilation and burning software packages do the job perfectly well without recourse to external hardware. My own view is that if you have a computer system then it's easier and better to record stereo mixes directly into the computer at 24-bit resolution (even if that means recording back to a spare stereo track on your sequencer) than to go via a 16-bit CD-R machine. CD-R is perhaps a better bet for those people who don't use computers, but be warned that not using a computer makes editing and compiling albums very difficult, and tracks recorded one at a time to CD-R can't be used as commercial CD masters unless they are later recorded in disc-at-once mode.

One final thing to bear in mind with CD-R and CD-RW media: they vary enormously in quality, so cheap unbranded discs are not the best bet for anything you want to archive (they tend to have a shorter life span), and their error rate also tends to be higher than that of quality branded discs.

Mixing To Your Computer

Mixing directly to the hard drive of your computer now makes a lot of sense, because if you have a computer then you'll need to get the audio in there for editing anyway. In the future, it's likely that many users will rely entirely on virtual instruments and effects, so creating a mixed file will be possible entirely within the DAW software.

Even so, my guess is that most current users have some external MIDI devices which they want to add into the mix, and though it is possible to record these as audio sequencer tracks, the most common option is still to use an external mixer to combine the computer audio interface outputs with those from the hardware MIDI instruments. In this case, the stereo mix from the mixer's main output would be fed back into the computer via the analogue audio inputs (or possibly the S/PDIF inputs if a digital mixer is being used) and recorded into the sequencer as the mix is played back, ideally as an interleaved stereo sound file on a stereo audio track. Common file formats are WAV, AIFF and SDII, and the better stereo editing and CD mastering/burning programs support the import of all these file types, with WAV being the standard for PC users and AIFF or SDII being more commonly used on the Mac platform.

Mix & Match

The requirements of mix recording have changed considerably over the past couple of decades and may do so again if surround sound becomes more widely adopted within the consumer audio market. It is now widely accepted that recording and mixing should be done to a greater bit depth than the delivery medium, though there is some argument over whether it is best to record at a high sample rate and then convert down to 44.1kHz at the end, or whether to work at 44.1kHz throughout. My own approach is to work at 44.1kHz throughout a project, but the jury is still very much out on this.

Mixing into your computer at 24-bit resolution is probably the best way to maintain signal quality if you run a computer-based studio, but making 16-bit CD-R backups would also be prudent, and unless you're recording music with a wide dynamic range in a very fastidious way, you're unlikely to hear any appreciable difference in the end result should you need to work from the CD-R.

For anyone not using computers, hardware recorders that use an uncompressed audio format provide the best results, and CD-R has the advantage that you can make copies of your work to pass on to record companies or friends, just by plugging the digital out of a regular CD player into the digital input of the CD-R machine. Minidisc and other compressed media are best avoided for work intended to end up on CD, but are fine for serious demos, independent releases of some types of pop music and, of course, for mixing music intended for Internet distribution. The options may seem confusing, but at least you can't say we don't have a choice!

Published April 2002