TAP-plugins
Tom's Audio Processing plugins
for audio engineering on the Linux platform

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## TAP Stereo Echo

This plugin supports conventional mono and stereo delays, ping-pong delays and the Haas effect (also known as Cross Delay Stereo). A relatively simple yet quite effective plugin.

### General information

Unique ID | 2143
I/O ports | 2 inputs / 2 outputs
CPU usage (44.1 kHz) | 0.4%
CPU usage (96 kHz) | 0.8%
Hard RT Capable | Yes
In-place operation | Supported
run_adding() function | Provided

### Usage tips

If you want to create a conventional mono or stereo delay, all you have to do is set the delay times and feedback ratios for the two input channels. If the feedback is set to zero, then only one delay of the input is created (as if a traditional tape delay was used). When feedback is greater than zero, the same bit of audio is sent back and delayed over and over again, with decreasing amplitudes. In this case the decay time is dependent of the feedback value.

When you turn the Cross Mode switch on, the feedback loops are being feeded by the delayed signal of the opposite input channel. A sound coming from the left input channel will be delayed by the Left Delay time value, but the output of the left delay ringbuffer will be sent back to the right input (instead of the left) with regard to the right channel's feedback setting. In Cross mode, a sound appearing at one input will be bouncing between the two output channels, hence the popular name "ping-pong delay". If you use this effect with high delay times and feedback values, even the dumbest listener will notice that your mix is stereo.

A third effect achievable with this plugin is the so-called Haas effect. This effect is founded by the following psycho-acoustic experience: if a sound reaches one ear, and the same sound reaches the other ear but with a time shift of 15-40 milliseconds, only one sound is heard, but with a spatial feeling. The Haas effect (also known as Cross Delay Stereo, which refers to the means by which the effect is created) is widely used by mixing engineers to avoid their mixes being "pan-potted mono", or to "stretch out" their otherwise mono guitar, vocal etc. tracks in space. This effect is a great alternative to reverberation (although it produces a noticeably different quality).

### How to create the Haas effect

It only makes sense to create the Haas effect on a mono track. On a stereo track, you should apply a cross or normal stereo echo instead.

To create the Haas effect on a mono track, follow these steps.

1. Switch on the "Cross Mode" and "Haas Effect" controls of the plugin. The "Haas Effect" switch will mute the second (right) input channel, because when applied to a mono track, the plugin receives the same mono data on both inputs and this would kill the Haas effect (this topic was discussed in the section about signal routing).
2. Set the "R/Haas Delay" time to 15-40 milliseconds. The bigger this setting, the wider the mono track will "stretch out". But if you increase the Haas delay above a certain threshold, the listener will begin to hear two separate sounds shifted in time instead of the Haas effect (which is actually the case at lower delays as well, but the feeling is something completely different).
3. Set the "R/Haas Feedback" near 100% (above 80% will do).
4. "Left Feedback" shouldn't be very large (stay below 50%) or the ping-pong delay which makes up the Haas effect will not decay in a short time, and this will create a very unpleasant sound.
5. Set the "L Delay" time as you see fit.
6. The sound of a mono track "streched out" in space with the Haas effect tends to have some directionality (the listener feels the sound source is a bit nearer to the right side than the left, or vice versa). If the sound of the track would fit into your overall mix better with the left and right sides swapped, you can do this by switching "Swap Outputs" on.
7. Mute the direct sound (set "Dry Level" to -70 dB). It is not needed in this scenario.

### Summary of user controls

name | min. value | default value | max. value
--- | --- | --- | ---
L Delay [ms] | 0 | 100 | 2000
L Feedback [%] | 0 | 0 | 100
R/Haas Delay [ms] | 0 | 100 | 2000
R/Haas Feedback [%] | 0 | 0 | 100
L Echo Level [dB] | -70 | 0 | +10
R Echo Level [dB] | -70 | 0 | +10
Dry Level [dB] | -70 | 0 | +10
Cross Mode | OFF | OFF | ON
Haas Effect | OFF | OFF | ON
Swap Outputs | OFF | OFF | ON

### Notes

The maximum delay time (which is currently 2000 ms) can be set to a greater value in `tap_echo.c` if needed. When activating the plugin, memory is allocated for a ringbuffer which is large enough to contain audio as long as this value. Because this amount of memory is proportional to the maximum delay, it is not desirable to set it to a very large value if you don't want to actually use it since you will be only wasting memory. (This waste will be temporary of course, since memory is freed when the host deactivates the plugin -- that is, when you remove it from the mixer/patchbay/whatever your host has.)