More Bits Please!

Let’s see how you can keep the sound of your tape (or digital file) intact on its way to the CD Mastering House.

Let’s discuss some digital do’s and don’ts.

The multitrack may require editing before mixing. After mixing there may be more editing of the 2-track or surround product before it goes to mastering. We have a number of articles on mixing and processing at the digido site worth taking a look at.

If you mix to analog tape, it’s best to make a safety digital copy, edit the analog (if necessary) with a razor blade, and send the original tapeto the mastering house. A 30 IPS, 1/2″ two-track tape contains a wide frequency and dynamic range, and is a superior recording medium. Some will argue that analog tape is more pleasant sounding than a digital recording (is that why are so many of us are nostalgic for the sounds of the 50’s and 60’s?). My essay called  Back To Analog  talks about those sonic differences. But the newer digital formats record at 24-bit or higher, at sample rates up to 96 Khz and beyond (though the benefits of 192 kHz are controversial considering the human ear can hear nominally to 20 kHz). We are living in very interesting (and expensive) times. My  Back To Analog  essay makes some comments about the sound of 96 Khz/24 bit digital audio, and the article  Audio Mastering  refers to some dos and don’ts about digital versus analog processing.

When mixing, be cautious about applying additional analog processors to the tail of your mix bus, since the mastering house, using high quallity monitoring and experience, can supply “just enough” warming or “sweetening” or whatever your mix may need to take it to that “finished quality”. At the least, send both versions to the mastering house, or send the mastering house your first mix for a listen/eval. Collaborate with your mastering engineer because mixing and mastering are a continuum.

What about digital copying? Digital copying is ok. But what about digital editing, level changing, equalization or other processing in the digital domain? We recommend that you avoid going down multiple dsp generations, especially to add processes which are better left to the mastering stage. Here are some of the reasons why…

Question Authority…

No processor (analog or digital) is totally transparent. Try to keep your fragile mix from going downhill before sending it to mastering by avoiding additional DSP generations. Those little bits can undergo a perilous journey through some of the digital processors and editors on the market. If there’s a DSP inside, suspect the worst until you know for sure. There are  some tests  you can perform on your digital processors and editors (or workstations) without expensive test equipment. These tests include linearity, resolution, and quantization distortion, common problems caused too-often by digital audio editors. Digital processors have gotten much better since I first wrote this article, but it still pays to do the minimum of tests (both listening and measurements) on any processor you intend to use on your entire mix bus!

Digital editors have gotten much better since I first wrote this article, but it still pays to check your editor for technical performance. Unity gain should produce a perfect clone and not add or take away bits. In addition, when editing, be aware that noise in your listening environment may fool you into thinking that your fades are fine. We have received mixes with truncated fades (where the audio sounds like it dropped off a cliff!), because the mix/editing engineer was not aware he was cutting off audio. We’ve received mixes with distorted audio on the fadeouts because the DAW the mix engineer was using was improperly dithered. We’ve received music with poor low-level resolution that is a shadow of its former self because the DAW the mix engineer was using had poor low level resolution. We’ve received music whose soundstage (stereo width and depth) appears to have collapsed, or recordings that have an indescribable “veil” over the sound compared with their sources. Here are some pointers that will help you avoid these problems:

Don’t wreck your digital mix…

• Always make a safety copy. Never send your only copy in the mail. These days with FTP, which is the original, anyway?
• If you would like to try some post-mix processes on your already-mixed file (for example, eq, compression, tape emulation) be sure to send both versions to the mastering house. Perhaps we have a better or superior-sounding method of getting where you want to go. It’s easy to fall in love initially with a squashed mix that later proves to be fatiguing and boring.
• Please mix for sound, not for competitive loudness. If you try to “compete” in level with any mastered product you will actually defeat your purpose! It is very hard for me to turn a squashed and overcompressed mix into a loud master. It’s actually a lot easier to make a loud master from a clean, dynamic mix. Do not worry if you think your mix is not as “hot” as a current release. If your mix sounds good when you turn up your monitor, this is all you need to do. Loudness-making tools require skill to avoid degradation or distortion, especially cumulative distortion from every cumulative stage of processing, when the recording hits the radio or the Internet. The mastering house has a lot of experience in this area (especially when it comes to making good-sounding iTunes masters!), so why not send a sample mix for a listen/eval after you’ve finished mixing the first song of the album.
• Always check the files you intend to send for mastering. If you made them via bounce (bounce to disk, aka “capture”), test your files by bringing the captures back into your editor and make sure you didn’t upcut a beginning or miss an end. Good advice is to add 5 seconds to heads and tails… better safe than sorry. A five minute check on your part can save hours of grief later on.
• Send the Unedited Original: Editing is like whittling soap, especially when it comes to decays, fadeins and fadeouts. We recommend you send the “raw” original, unfaded material to the mastering house (along with a good written log of where to find the cuts). If you have suggestions or ideas on fades, you can send two versions of the song, or example fades. If you want to perform crossfades, it’s better to leave them for the mastering house, because in mastering we may want to apply different equalization or processing on each song that is crossfaded. You can send crossfade examples so the mastering engineer knows exactly where you would like your crossfade and he’ll emulate or duplicate your fade. Also, fades which are performed in front of compressors can sound very different than fades performed after compression, so we may have to manipulate the fade to get the same smooth fade that you conceived. Plus, there are things we can do that you may not have considered; not every fadeout is necessary. For example, I’ve got some tricks that can create real-sounding endings on tunes that everyone thought had to be faded. There’s even a bonus in sending the original raw mixes, as we now have available outtakes, alternate mixes (vocal up, vocal down, etc.) or other sections the mastering engineer can use to repair noises or problems you may not have noticed. The mastering engineer will order the tunes, carefully smooth fade-ins or fade-outs, place black or roomtone between the tunes, in extremely efficient time. Plus, at the mastering studio, each fade-out or level will be controlled with  dither,  a topic worthy of discussion.
• Levels: Peak levels ideally should not exceed -3 dBFS on your meters. Even if you are absolutely sure of the accuracy of your DAW’s meters, do not exceed -1 dBFS because most DAW’s meters do not measure true peak, and true peak levels can exceed 0 dBFS on a standard DAW’s meter. Standard DAW meters do not reflect intersample peaks which can be OVER 0 dBFS even if not shown on your meter. At 24-bit, you do not have to worry about signal to noise ratio and you will get a better result with a lower level and leaving some headroom for the mastering house. You would have to drop a 24-bit recording by 48 dB to reduce it to 16-bit resolution, so there’s a lot of room—use it!
• Noises: Alert the mastering engineer to any noises that bother you (note the time from first downbeat or from the beginning of your file). And we may be able to remove them with our noise-reduction processors, which include Algorithmix, TC Backdrop and Izotope. If the musicians talked before the ringout was over, or the bass player dropped his bow (shit happens), or the assistant stopped the recording before he was told, we can apply some of those techniques I mentioned to add convincing tails to a song that are indistinguishable from real life, and sometimes even better! In addition, Algorithmix (Renovator), Izotope and Cedar make cleanup tools that can remove many types of noises from mixed material. It’s a judgment call which noises are better left to be repaired in the mastering. If you can repair a noise by muting or fading down the instrument that makes the noise during the mixdown without creating an artifact, it’s better for you to remove the noise. For example, noises made by a vocalist during a decay, where you can fade down or mute the vocalist’s mike. Conversely, some noises might sound good if left in, producing a “relaxed, easy going feel” to an album. This includes countins, sticks, verbal comments by the musicians, and so on. Tell us the noises you like to keep, and we may find other noises that help to glue the album together.
• If you would like to perform some complex editing prior to sending the material, TEST YOUR EDITOR with a bitscope to ensure that the output equals the input. Listen carefully for degradation of soundstage width and depth, graininess, increased brightness or hardness. Listen on the finest reproduction system possible, or these changes may be perceived as too subtle and you won’t know you’ve ruined your material until it’s too late! You’re welcome to send us a preliminary mix before you mix all your tunes. We will check it for tonal balance and for digital errors before you proceed.
• Keep the bits. Cumulative digital processes if improperly performed can be very degrading to sound. The reason (and many engineers are not aware) is that almost every DSP computation adds additional bits to the wordlength. The wordlength can increase to 24-, 56-, or even 72-bits. The right thing to do is keep your newly “lengthened” words as long as possible, until the final stage, where we will dither them down to 16-bits for the CD. 16-bit dither should be reserved as a one-time only process at the end of the chain.
• What makes the CD mastering house different?
  The analog and digital chains at a good mastering house have been vetted for audio integrity (low distortion, jitter, high resolution….). Signal paths are kept to a minimum. If the mastering engineer employs digital processing on your tape, he/she will endeavor to keep your tape in the 24-bit domain (or even longer) until the final stage. When properly applied, high resolution processes maintain a degree of warmth and space that is hard to believe. And that’s why it can sound so good! Use the Mastering House like a mothership, ask us any questions you like, because our sole job is to make your recording the very best it can sound.

Did you know that the S/PDIF output of the older Yamaha mixing consoles is truncated to 20 bits? Now how did I know that? Because I tested it! And you can, too, with some very simple equipment. There are some legitimate reasons why Yamaha made that choice, although I do not agree with them. This means that if you want to get all 24 bits out of your older Yamaha digital console, you must use the AES/EBU output. There are simple ways to adapt the Yamaha’s AES/EBU output to the S/PDIF input of your soundcard, and this will preserve all the bits. Many (if not all) soundcards that work at 24 bits accept the 24 bits on their S/PDIF inputs.

A Bitscope You Can Build Yourself

The first defense against bugs is eternal vigilance. Listening carefully is hard to do-continuous listening is fatiguing, and it’s not foolproof. That’s why visual aids are a great help, even for the most golden of ears. In the old days, the phase meter was a ubiquitous visual aid (and should still be a required component in every studio); our studio also uses a product we call the “digital bitscope”, that is easy and inexpensive to put together. It’s not a substitute for a $20,000 digital audio analyzer, but it can’t be beat for day-to-day checking on your digital patching, and it instantly verifies the activity of your digital audio equipment. Think of it this way: The bitscope will tell you for sure if something is going wrong, but it cannot prove that something is working right. You need more powerful tools, such as FFT analysers, to confirm that something is working right. 

However, the bitscope is your first line of defense. It should be on line in your digital studio at all times. You can assemble a bitscope yourself–see  The Digital Detective.  If you’re not a do-it-yourselfer, Digital Domain manufactures a low-cost box that can be converted to a bitscope with the addition of a pair of outputs and a 2-channel oscilloscope. Our bitscope is always on-line in the mastering studio. It tells us what our dithering processors are putting out, it reveals whether those 20-bit A/D converters are putting out 20-bit words, and it exposes faults in patching and digital audio equipment.

Some Simple Sound Tests You Can Perform on a DAW
With the output of my workstation patched to the bitscope, I can watch a 16 or 20-bit source expand to 24-bits when the gain changes, during crossfades, or if any equalizer is changed from the 0 dB position. A neutral console path is a good indication of data integrity in the DAW. After the bitscope, your next defense is to perform some basic tests, for linearity, and for perfect clones (perfect digital copies). Any workstation that cannot make a perfect clone should be junked. You can perform two important tests just using your ears. The first test is the fade-to-noise test, described previously in my  Dither  article.

The next test is easier and almost foolproof-the null test, also known as the perfect clone test: Any workstation that can mix should be able to combine two files and invert polarity (phase). A successful null test proves that the digital input section, output section, and processing section of your workstation are neutral to sound. Start with a piece of music in a file on your hard disk. Feed the music out of the system and back in and re-record while you are playing back. (If the DAW cannot simultaneously record while playing back, it’s probably not worth buying anyway). Bring the new “captured” sound into an EDL (edit decision list, or playlist), and line it up with the original sound, down to absolute sample accuracy. Then reverse the polarity of one of the two files, play and mix them together at unity gain. You should hear absolutely no sound. If you do hear sound, then your workstation is not able to produce perfect clones. The null test is almost 100% foolproof; a mad scientist might create a system with a perfectly complementary linear distortion on its input and output and which nulls the two distortions out but the truth will out before too long.

If the workstation is 24-bit capable, and your D/A converter is not, you may not hear the result of an imperfect null in the lower 8 bits. Use the bitscope to examine the null; it will reveal spurious or continuous activity in all the bits and tell you if something funny is happening in the DAW. Even if your DAC is 16 bits, you can hear the activity in the lower 8 bits by placing a redithering processor in front of your DAC.

Use the powerful null test to see whether your digital processors are truly bypassed even if they say “bypass”. Several well-known digital processors produce extra bit activity even when they say “bypass”; this activity can also be seen on the bitscope. Use the null test to see if your digital console produces a perfect clone when set to unity gain and with all processors out (you’ll be surprised at the result). Use the null test on your console’s equalizers; prove they are out of the circuit when set to 0 dB gain. Use the null test to examine the quantization distortion produced by your DAW when you drop gain .1 dB, capture, and then raise the gain .1 dB. The new file, while theoretically at unity gain, is not a clone of the original file. Use the null test to see if your DAW can produce true 24-bit clones. You can “manufacture” a legitimate 24-bit file for your test, even if you do not have a 24-bit A/D. Just start with a 16-bit or 20-bit source file, drop the gain a tiny amount and capture the result to a 24-bit file. All 24 of the new bits will be significant, the product of a gain multiplication that is chopped off at the 24th bit. You’ll see the new lower bit activity on the Bitscope.

Let’s discuss the use of digital consoles with digital recorders. Knowing how to use this gear really separates the men from the boys. Digital consoles suffer from the same wordlength and truncation problems as DAWs. Truncation without redithering is always bad, but depending on where you truncate, the result can be sonically benign, or very nasty. For example, truncating a 24-bit A/D to 16 bits is relatively benign because most mike preamps are noisy enough to provide some dithering action. But using a DSP to drop gain only .1 dB in a console and then truncating the output to 16 bits is very damaging, shrinking soundstage and producing harsher sound. Be aware of these facts when using digital consoles with digital recorders. Always use dither to reduce the console’s long wordlength to the recorder’s wordlength. If your digital console does not have dithering options, you’ll be better off with a very high-end analog console. That’s one of the things that separates the higher priced digital consoles from the cheap ones. Cheap digital consoles do cost–you pay in reduced sound quality.

There’s an engineer on the leading edge, who had been working with 24-bit recording and a digital console, but reverted to a purist-quality analog console when he upgraded his converters to 24 bits. He found he got better-sounding results mixing live sources in analog and then feeding the 24-bit A/D than by starting with A/D’s and feeding a digital console. It takes a very special digital console to preserve 24-bit quality; it’s also difficult and expensive to design an A/D converter that retains high resolution inside the polluting environment of a digital console.

At Digital Domain, we encourage clients to send us 24-bit files or 32-bit float files at the highest sample rate you are working at. See my article  Preparing Tapes and Files for Mastering  for descriptions of all the new high-bit formats.

DAWs, digital tape recorders and digital consoles affect sound. Use these tools properly and your music will sound better. Mastering houses thrive on high-resolution sources. Consider the choices and send the best source you can for mastering. Manufacturers–Give Us More Bits–and please, make them compatible!