Phase Aligned Tape Comp Combinator

Wednesday, April 22nd, 2009 | 11:18 am and filed in Reason, Combinator.

/images/journal09/phase-distinvtn.jpg

In case you didn’t know, the Scream 4 Tape Compression algorithm induces somewhat of a phase inversion to signals passing through the device. As the photograph of the oscilloscope indicates, It’s not a perfect phase reverse because the signal is also being processed by the effect, but it’s enough to cause some problems if you’re not aware of this characteristic.

In order to determine the linearity of the phase response of the Scream 4, I broke out the old oscilloscope and wired up a stereo jack to a pair of bnc connectors to monitor two channels coming from my workstation. In Reason, I created a basic configuration of a Thor being self-triggered with the internal sequencer with the analog oscillator and no filters. The Thor Synthesizer was connected to a mixer with a Scream 4 inserted on the right channel only. The left channel was unprocessed.

Running sine waves through the Tape Comp reveals that there is indeed a drastic phase inversion. This comes as no surprise. Running square waves and sawtooth waves reveals that the rising and falling edges are modulated by the effect to a point where higher frequencies will pass, but low frequencies are inverted. Two traces on the scope displayed the left and right channels separately. In the photo you can see that the top trace moves in the opposite direction of the bottom, indicating the waveforms are inverted.

If you’re using the Scream 4 Tape Comp as an insert effect on a loop or synth device, the phase inversion may probably go unnoticed, however if you decide to use it as a Parallel effect to thicken basses or drums, the process will actually do the opposite. There’s an example file posted below.

There are two ways of re-inverting the signal of the Scream 4 tape comp signal. Either insert another Scream 4 Tape comp or use a Thor Polysonic Synth. Phase inversion with Thor is accomplished by routing audio inputs to corresponding audio outputs in the programming matrix, and setting the amount value to -100 for the routing. I’ve made patches using both techniques and made them available below. The archive contains the following patches:

Phase Aligned Tape Comp S4.cmb uses a secondary Scream 4 to correct the phase. Sounds a bit vintage - dull & portly.
Phase Aligned Tape Comp T.cmb uses the Thor inversion technique. Sounds pretty clean.

Download the Phase Aligned Tape Compressor Combis

The Rotary Controls are assigned to the following parameters on the Scream 4: Damage Control, Speed, Compression, and Master. These correspond to the main controls one uses when applying the tape compression algorithm. The S4.cmb has an additional control for rolling off a bit of hi frequencies induced by chaining the effects.

Additionally, I’m posting a Reason song file that demonstrates the use of the scream 4 as a parallel effect and the potential issue the phase inversion causes.

http://peff.com/reason/download/rps/ParallelTapeComp.zip

5 Responses to “Phase Aligned Tape Comp Combinator”

  1. peff.com » Blog Archive » Scream 4 Waveforms Says:

    […] Tape Algorithm - See the related post […]

  2. meowsqueak Says:

    I’ll have to check this out with TrueRTA - I’m interested to know at what frequency the inversion occurs.

    So, do you think this is a bug or a feature of the tape algorithm?

  3. meowsqueak Says:

    I misread something you said - I see now that the complete signal is inverted. However the cancellation effect will be most noticeable with lower frequencies.

  4. Peff Says:

    it’s been there since the inception, so by now it’s a feature by default :) It can be useful for a specialized filter.

  5. Distortion « Re-Wiki Says:

    […] Scream Tape Algorithm Phase Inversion by Off White Noise Peff points out here that the Scream4 ‘tape’ algorithm has an effect of inverting the output waveform at lower frequencies. This can have the effect of canceling with the original signal if mixed back (e.g. as a Send Effect). He posts a couple of solutions and a good example that illustrates this “feature”. […]

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