Analog Versus Digital Signal Processing (dynamics)

Limiters (peak limiters, protection circuits)

Most common is a digital plugin. Plugins tend to be much faster, cleaner, and have less overshoot than what you get in the analog-domain.

Compressors

When you look at an equipment roster of a high-end mastering studio, a compressor will more likely than not be seen in any of the studio’s analog gear. It seems that DSP (digital signal processing) plugins do an excellent job of recreating the dynamic-range control that happens in an analog compressor, yet they don’t quite seem to sound just as good.

What could be the reasons for this? Let’s speculate a little.

When audio comes out into the analog-domain, you get added distortion and noise. These are not necessarily characteristics that will be programmed into the digital circuits (or, algorithms). As a result, you get a subtly different overall presentation of the sound.

The detection circuit (the device used to tell the compressor when to compress the audio passing through it) is what really drives the action of a compressor. Another possibility, which was proposed to me by George Massenberg, is that sample rate for the detector circuit in a digital compressor needs to be much higher than the typical sample rates we are using now, because of the nuances that you typically get at the output of a compression stage. 44.1kHz may be sufficient for the audio passing through the compressor, but it may not present enough detail for the audio that is feeding the detection circuit for the compressor to do as good a job as its analog counterpart. It is speculation, but it is an interesting point to consider.

A lot of the time you will find mastering engineers using an analog circuit not because they’re going to use an EQ to equalize, or a compressor to compress, but because there is something about the filtering that takes place when running audio through that analog gear that changes the sound in a desirable way. So, a great compressor may be used not to compress, but simply due to the tone-shaping sound that is imparted to the program. This seems to be a common factor missing from many current DSP equivalents.

Tape to the rescue?

In the course of mastering a record, the obvious first step is to listen to the record, and listen to the client talk about the record in order to decide what, if anything, needs to be done to change it.

The 'what' in that sentence can include many things:

- adjusting relative volume
- adjusting absolute volume overall
- deploying dsp based processing (eq, compression, reverb, etc)
- deploying analog processing including solid state and tube based amplifiers

and then there's the more obscure tool such as running the audio through an analog tape record/play pass. This particular technique is something that has a reputation for being something like the 'Holy Grail' of audio in some circles. You'll see it played out in the marketing literature of modern audio systems that include tape simulations. Something like "that vintage tape sound" or "that professional sound you get from tape" will appear that seeks to glorify the process and imbue it with a sense of desirability such that 'if you do this you will undoubtedly elevate and improve your recording'.

It's hard to imagine resisting such a tool. Just turn it on and something good happens? Bartender make mine a double!

OK, ok, so once we come to our senses it's worth thinking about what tape does to audio. The short of it, in a word, is it creates non-linearity. It changes things. OBviously it adds noise (hiss) that is generated at the record 'head'. A small bit of distortion is added as a result of the process. It also changes the dynamic range, especially in the peaks as the signal applied to tape increases in level. The peaks are restricted, 'rolled off' if you will, as the level gets hotter and hotter. The frequency response changes. Tape machines do not record perfectly 'flat' from 20 Hz to 20kHz. If you chart the frequency response of a tape machine you will notice slight deviations from 'flat' of about 1/2 dB in a great machine and more in a good machine. A particularly noteworthy area where non-linearity occurs is in the bass.

There's a phenomenon called the 'head-bump' which refers to a resonant frequency of the record head itself. It will add emphasis in the bass and interestingly, the emphasis will change with tape speed. At 30 inches per second, usually this head bump is around 50 HZ. At 15 ips it is around 25 (1/2. Below this resonant peak the tape machine will roll off bass very steeply, almost like a high pass filter.

So enough tech talk. Suffice it to say that the process changes the sound substantially. So, let's say you have a mix you are REALLY happy with. Do the changes I mentioned above sound appealing? I hardly think so. Creating a bump in the bass, adding noise (and thereby losing reverb and imaging) and losing transients are going to change your mix in a way that is hard to control. These changes are not a panacea.

However....if your mix is too bright and spiky, lacking in bass 'punch' and definition, already 'noisy' in a way that a little noise added won't be a problem....then maybe tape is the way to go.

That's what happened last week when I mastered a project. Tape saved the day, adding desirable tonal color and compression in one fell swoop in a way that I couldn't have achieved any other way....maybe the ads should read -

"Tape might be the perfect mastering solution when your mixes are not"......


PS - in case it's not obvious, the piece above is written in the context of mastering. When considering recording or mixing where you can adjust, react to and minimize the sonic impact of tape, the choices and decisions might be different.

Analog everything

This week I am working to archive and renew the theme Music to the PBS Show "Frontline' composed by Mason Daring. It's a stalwart PBS newsy long form show and the theme was recorded in the early 80's. All analog. I am hearing mono and stereo versions, 15 IPS (inches per second) some encoded with Dolby A. Mostly Ampex 456...and it held up REALLY well! All analog musicians too. French Horn, Oboe, Trumpet, Strings, Timpani.

IT remarkable how well the music holds up. The performance is really quite excellent, mixed originally by Glen Berger at Blue Jay in Carlisle, MA. The sound holds up really well also.

What a blast!