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Analog Channel Applications

The McDSP Analog Channel plug-in was designed to emulate the sounds of analog mixing consoles (busses), tape machines, and tape media. The notion that a purely digital production can sound like an analog one is great for ProTools users needing to satisfy the myriad of client needs. While Analog Channel is an ‘old’ plug-in (first released in 2001, where it received a TEC Award nomination), it is still used today in many Pro Tools systems the world over. In this issue we’ll look at how to use Analog Channel in some ‘analog applications’.

AC101

Analog Channel’s AC101 is a class A gain stage – the premium amplification design in the analog world for audio production. AC101 gives control over many parameters previously unavailable to the common user. This flexibility allows for a number of ways to improve a mix.

Class A Glue

Many digital audio workstation users have noticed how their mix inside the digital environment can be altered significantly when bussed through an analog console. The differences are in part due to the amplification stages in these consoles. Where the digital system reproduces its dynamic range in a linear fashion (with the awful digital clip at the top of its range), the analog circuit has a built-in limiting action for high signal levels. This allows signal peaks to hang on slightly longer as they become slightly compressed. This ‘glue’ action can affect the listener’s perception of the music, particularly in the percussive elements. The stereo field can even become different as signals panned from center are ‘glued’ during high peaks. In the digital mixer this effect does not exist.

Using AC101 as a multi-mono plug-In on the stereo buss, try out the “Console 1”, “Console 2”, and “Console 3” presets that come with the plug-In. The differences can be extremely subtle, and will only occur during signal peaking. But note how the stereo field and overall feel of the music can change with the plug-In engaged.

But there is always the question of how ‘hard’ to hit the AC101 plug-in. A good way to evaluate what the AC101 plug-in is doing is to observe the three-state LED next to the Drive control. When the green LED is lit, no signal saturation / distortion / compression is occurring. The middle and upper LEDs indicate some amount of saturation, etc. is occurring. Increasing the Drive control until the middle LED is lit during signal peaks is a good setting generally. If the middle LED is not lit even when the Drive control is maxed out, increase the Input level control until this occurs.

Digital Clip Removal

The AC101 plug-In can also correct the occasional digital clip in recorded tracks. By selecting the fastest attack time (to insure each clip is captured), the AC101 plug-In can smooth out the digital clipping and make the event appear to more like distortion on an analog mixing buss.

Varying the Release and Comp controls is a good way to audition different AC101 ‘effects’ on the clipped signal. Faster release times will make the clip ‘recovery’ occur quickly, and avoid pumping effects of moderate to long release times, although sometimes a longer release time is subjectively better. Be sure the Comp control is at least at 50% of its maximum value so the AC101 plug-in has a good range in which to smooth out the signal clip.

Threshold-less Compressor

Unlike a compressor, the AC101 algorithm has no internal threshold. This means its transition between doing ‘nothing’ and ‘something’ is seamless. For this reason, the AC101 plug-in can be used to achieve some subtle compression-like effects. Here’s how…

Use the Drive and Comp (short for Compression/Saturation/Distortion Curve – dang I should have just called it ‘Curve’) controls to vary the amount of effect – in this case the amount of ‘compression’. Set the Curve control at its maximum setting to make the changes as audible as possible. Now vary the attack and release controls as you would normally when using a compressor. Release ranges between 100 and 300 msec can give a more compressor like sound. I find 300 to 600 msec are a little more ‘natural’ sounding. Anything under 100 msec is really pushing your luck as far as getting too much distortion, but field use has shown some folks like to go as low as 10 msec, hence the control range. Release times greater than 600 msec tend to make AC1 hang on too long for me, but when the overall effect is desired to be as unobtrusive as possible (i.e. change so slowly the casual listener does not hear the effect), it can work. For attack ranges, 0.1 msec to 0.5 msec are good for a wide variety of material. However it is important to note that really fast attack times can cause some buzzing. I keep the attack around 1 to 3 msec to avoid the bass-buzz effect. Also note how you can use slower attacks (greater than 3 msec) as the Comp control is increased to its maximum. When the compression/saturation/distortion curve has enough ‘bend’ in it, there is more headroom for overshoots to occur, and hence a slow attack is ok. But watch for output clipping!

AC202

Analog Channel’s AC202 comes with a variety of playback heads to realize different ‘tape experiences’. Although each playback head’s use does not need to be restricted to certain applications, there are some combinations of settings and source material AC202 is particularly well suited.

Drums

Two great uses AC202 on drums are the elimination of rumble at extremely low frequencies, and the separation of the bass and snare drums sounds. The playback head system in AC202 can address these sonic issues.

Setup a drum sub mix, insert the AC202 plug-In on the master fader of the sub mix. Starting with the factory default preset settings, increase the Bump control to its maximum setting. This will give roughly 6 dB of gain at the frequency value indicated by the Freq control. Move the Freq control around until the sweet spot (region with the most signal content) of the bass drum. Note how alternating between the different playback head types affects the tone of the bass and snare drums. The ‘USA-A’ and ‘USA-M’ playback heads also can do a good job of separating the bass and snare drums. The subtle gain reduction between the main bump and secondary bump of the playback head response helps isolate each sound from the other. Furthermore, the playback head response will eliminate low frequency content below the bass drum. This is a necessity in improving the low end of any drum mix (or mix in general!). The punch of the bass and snare is preserved while low frequencies below the signal content of the drum kit itself.

Guitar

The AC202 Plug-In can further the pursuits of THE guitar tone with the playback head section. Insert the AC202 Plug-In on a guitar track (distorted or clean). Starting with the factory default setting, select the ‘USA-M’ playback head. Using a Freq control value of 100 Hz and a Bump of 100 (6 dB), a subjectively pleasing EQ curve is setup. Over-biasing at 15 ips (even 7.5 ips) will give a tone closer to that of many guitar amplifiers and speakers. Use a Release control time of at least 1.0 seconds to avoid any pumping effects (unless desired).

Master Fader

Dumping the entire Pro Tools 2-track master into a tape machine was very common when Analog Channel first came out. Many users discovered the great sounds of those analog reel-to-reel machines by using the AC202 plug-in. Still, the flexibility of AC202 can sometimes prevent users from finding that elusive ‘sound’, so here are some tips on how to use AC202 on a master fader.

For starters, put AC202 on the master fader as you start mixing. Many ‘old school’ folks like to mix with a compressor on the master, and this idea works well for AC202. There are a variety of presets that come with Analog Channel, but here are some good starting settings. I like the USA-A (Ampex) or USA-M tape head types, with about 20 to 40% head bump, at a roll off frequency between 30 and 60 Hz. The roll off frequency should be auditioned on the drum tracks mostly, as the head bump will be accenting the kick from the drum kit, and the bass instruments to some extent as well. Once a sweet spot is found (i.e. that kick really starts to resonate) the head bump can be varied to taste. Try 15 or 30 ips tape speed for most music styles – the 7.5 ips speed is a really retro kind of sound! The vintage tape formulation is derived from Ampex 456 (rock and roll!) tape, while the modern tape formulation is from GP9 (high dynamic range/low distortion) tape. The Bias control sounds great anywhere from 0 to about 6 dB. More than 6 dB can be excessive, but always experiment with the full range of any control at least a few times before settling on a good setting.

The visual monitoring of the gain reduction is key to seeing the AC202 is producing some of the tape compression kinds of effects you’re most likely looking for. Fortunately the AC202 provides simultaneous metering of the inputs, gain reduction (GR), and outputs.

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