Indistinguishable From Magic
Greg Simmons is the Founding Editor of AudioTechnology magazine, and looks forward to a time when recording musicians will look at an old interface and ask, “What does that knob marked ‘gain’ do?”
In the early ‘60s Arthur C. Clarke wrote: “Any sufficiently advanced technology is indistinguishable from magic.” Thirty years later I was baby-sitting a fully-configured CEDAR (Computer Enhanced Digital Audio Restoration) system; a great opportunity because my work involved restoring audio from tape, vinyl and 78rpm discs. Pre-dating iZotope’s RX and native processing by at least a decade, CEDAR was a PC-based system loaded with DSP chips that offered real-time de-noising, de-clicking, de-crackling, etc. It was amazing, and I wondered how to recreate it using the audio building blocks I was familiar with: amplifiers, filters and dynamics processing. I didn’t even know where to start. It was, essentially, indistinguishable from magic.
I asked CEDAR’s Gordon Reid to explain how it works in terms of the audio building blocks I was familiar with. After some failed attempts, he said, “I can’t explain it in those terms because CEDAR doesn’t ‘see’ an audio signal that way. It just sees information; it looks for patterns in that information and responds to them.” It was my first experience of ‘post-analogue lateral thinking’…
Fast forward to early 2017. I’m poring over a thread that’s unjustly ridiculing the latest field recorders. Supposedly well-informed professionals were dismissing them outright because the limiters were implemented in the digital domain, meaning they were after the converters and presumably too late to prevent overloading. “Ridiculous!” they scoffed. I’d been using these newer-generation field recorders with no limiter problems, and felt compelled to wade in with a dose of ‘post-analogue lateral thinking’.
OUTSTANDING IN THE FIELD
A single channel of a field recorder consists of a mic preamp, a limiter and an AD converter. In a contemporary field recorder these components form an integrated system with no user-adjustable level controls between them. Therefore, with no limiting taking place, monitoring the converter’s output is as valid as monitoring the preamp’s output. So far, so good…
There are numerous ways to make the digital limiter work, but I’m going to focus on the one I find the most interesting. In contemporary field recorders the preamp’s gain is digitally controlled by a rotary encoder or up/down buttons. The digital limiter monitors the output of the converter but applies the gain reduction directly to the digitally-controlled mic preamp. It protects the converter from clipping without any extra processing in the signal path. It is superior to the analogue equivalent in many ways, with one caveat: latency. If the internal processing is fast enough to match or better the attack time of an analogue limiter, it’s a superior solution and another good example of ‘post-analogue lateral thinking’.
For years, conventional thinking about AD conversion said there was no point going beyond 24-bits because we couldn’t make a converter that delivered 24-bit performance in terms of dynamic range. The rule of thumb for the dynamic range of a linear PCM system is simple: 6dB-per-bit. Not so long ago we were struggling to get 20-bit performance out of a 24-bit converter. At 6dB per bit a converter with 20-bit performance offers 120dB of dynamic range, which is less than a typical condenser microphone (e.g. 125dB for a Neumann KM184). One of the goals for manufacturers whose products are intended for situations where you only get one chance to record it (e.g. live performances, location recording) is to make a converter with a dynamic range that exceeds that of the microphones. It would be one less bottleneck in the signal path, and bring us closer to a situation where setting mic gain before recording will be an option rather than a necessity.
GAIN FIRING RANGE
In this issue, Stephan Schutze reviews Zoom’s F6 field recorder which, like Sound Device’s MixPre II series, offers the ability to record in ‘32-bit float’ format. It’s a different format to linear PCM so we can’t apply the 6dB-per-bit rule, but the 32-bit float format offers huge dynamic range – way more than a microphone could ever deliver. To achieve this they use a technique known as ‘gain ranging’. Instead of using one preamp and one converter on each mic input, they use two. Both preamps have fixed gain. One preamp/converter combo is optimised for low level signals, the other for high level signals. The outputs of both converters are combined through DSP to create a 32-bit floating point signal that is essentially impossible for any contemporary microphone to drive into clipping. Gain-ranging isn’t new — you’ll find it in the AES42 digital microphones from a decade ago — but it’s another example of ‘post-analogue lateral thinking’.
The F6 offers recording in 24-bit or 16-bit linear PCM format, along with the option of recording the 32-bit float signal. I’m willing to bet that the linear PCM signal is derived digitally from the 32-bit float signal, and that the knob used to control the recording gain is placed after the converters — a gamble backed by the fact that it affects the recorded level of the linear PCM signals but not the 32-bit float signal. Whatever the case, if you’re recording in 32-bit float mode from a microphone source there is no need to worry about gain anyway because you cannot overload the preamps or converters.
POST ANALOGUE WORLD
Marcel Gnauk of Free To Use Sounds (see ‘Making Free Pay’) recently shared an F6 recording of a passing train, along with screen dumps of waveforms. The whistle blows and drives the 24-bit signal well into clipping. Thankfully, he was also capturing the 32-bit float signal and was able to recover the recording. It gets a bit gritty at the peak when the mic’s diaphragm reaches its maximum SPL, but that has nothing to do with the F6. Yet another example of ‘post-analogue lateral thinking’.
So where is this leading? Zoom’s tiny F6 contains six ‘gain-free’ inputs. Imagine eight of them in a desktop interface, perfect for the non-technical recording musician. Imagine 24 in a rack-mounting interface, ideal for recording live performances. Worrying about recording levels, pad switches and limiters will be a thing of the past. For those who don’t understand the technology behind this ‘post-analogue lateral thinking’, it will be indistinguishable from magic.