Tuesday, 21 May 2019

Ping Pong Delay - Re-imagined differently! (Plus M4L Audio switches)

My first AUDpiPOde (Ping Pong Delay) M4L device had some protracted problems, and hopefully the additional tweaks for version 0.04 should fix them. But in the course of learning more about how the original (and soon to be deprecated) Ableton Live Ping Pong Delay actually worked, I realised that I had re-imagined it along the wrong vector, and that my 'all stereo' approach had too many differences to the original. The band-pass filter also proved to be a significant challenge, and dropping it again restricted the flexibility. (...Now, I'm not the greatest fan of 'muddy' echoes, but they are a defining characteristic of some vintage tape echo units...)


So, here's almost the opposite of AUDpiPOde - it uses a tapped delay line, mixes the channels into a mono signal for the echoes, and puts back the 'far-from-perfect' band-pass filter emulation made up from a high-pass and low-pass filter (and I need to learn more about filters in Max!). It uses a different 'freeze' method (as well as the ones I added), and it provides a 'Thru' button to bypass the filters. I also tweaked the delay routing so that you can have 'Echo in the Left channel first, then the Right channel' or 'Right channel first, then the Left channel' - which is quite striking if you are used to the 'feel' of the original!

And the name? AUDpiPOde-A, of course!

Making audio switches

One of the traps for people who are trying to learn Max are the large variety of switches. There are simple switches, complex matrix switches, routers and the names are sometimes different if audio is being switched. I'm still gathering information for a guide, but in the meantime, here's what I've been doing to build custom audio switches...

LR 'Left first' 'crossover' toggle switch

The 'Echo in the Left channel first, then the Right channel' or 'Right channel first, then the Left channel' switch functionality is a good example. All that needs to happen is that the two outputs from the delay (the middle tap and the end) need to be routed to the Left and Right channels, in the two possible combinations.


This is the output stages of the AUDdiPOde 'Ping Pong Delay' device, with, from top downwards, the feedback rotary control, the 'LR' echo order control, and the Dry/Wet rotary control. For the Feedback control, you can see how the line~ object is fed with pairs of '<new value> 50ms' values for each new line segment, so that the multiplier that does the feedback only gets values that change reliably slowly (each new value takes 50ms to happen) and so the amplitude of the feedback signal doesn't jump suddenly.

Just below the feedback code, the 'LR' box is where the interesting switching takes place. The 'p cross_mr' object has two stereo inputs (from the tap and the final output of the tapped delay line) and two stereo outputs (which go to the Dry/Wet' balance control library object. The 'LR' toggle switch controls the 'p cross_mr' object, and all it does is change between the default 'Left In to Left Out, Right In to Right Out' switch setting to the alternate 'Left In to Right Out, Right In to Left Out' which reverses the channel ordering. So in the default 'Left first' position, the 'LR' switch controls the 'p cross_mr' object so that the tap output of the delay line goes to the Left In of the 'cross' switch and comes out of the Left Out, which means that the tap output is heard in the Left channel first, whilst the final output of the delay is routed to the Right channel and so is heard later. When the 'LR' toggle is in the 'Right first' position, then the 'cross' switch routes the tap output of the delay to the Right channel where it is heard first, and the final delay output is routed to the Left channel, where it is heard later. So the 'cross' switch connections are either straight-through, or crossed-over - hence the name.

I don't think there is a standard MaxForLive switch that does this 'out of the box', so I made the 'cross' switch:

  
There are only two objects used inside the 'cross' switch - gate~ objects, which are just on/off switches for audio signals, and one of those arcane 'not quite obvious' special-purpose modifier objects: '!- 1', which inverts a 0 or 1 control value. So 0 becomes 1, and 1 becomes 0 - it is an 'inverter' for  control values. If you replace the two left-hand gates with through connections (closed switches) then you can see that A goes to X, and B goes to Y. Whereas if you replace the two right-hand gates with throughs then A goes to Y and B goes to X, and so achieves the straight-through or
crossover switching.


In physical hardware, then crossover switches like this are very easy to spot when you look at the rear  wiring of a front panel - the inputs go to the centre common part of the switch, whilst the outputs come from one of the outer pairs, and two wires cross over the outer pairs - so you can see at a glance that it is a crossover switch!

Fade switches

If you just switch from one audio signal to another, then you get sudden changes in the value of the signal, and these can cause clicks in the audio. To avoid this happening, one technique is to 'dip' the audio volume as you do the switching, and then restore it afterwards. This approach is used in AUDpiPOdePLUS, the 'performance-oriented' freeze echo device.

For this technique to work, then a sequence of operations need to happen in the right order, and at the right times. Here's the sequence:

1. The volume is at maximum.
2. trigger for the switching occurs. In the AUDpiPOdePLUS device, this happens when one of the 'Freeze' mode buttons is clicked.
3. The volume starts the fade downwards to zero.
4. The volume stays at zero for a few milliseconds, whilst the switching takes place.
5. The volume starts to fade back up.
6. The volume reaches maximum level again.

Once again, there wasn't a standard pre-prepped switch for the 'Freeze' mode button signal routing, so I made my own. It uses the 'constant volume' technique from a previous blog post, plus the 'gate~' switching describes earlier, with added 'fading' to dip the audio signals in and out at the right moments.


The three 'Freeze' modes are A, A+B, and B, which correspond to 'Tap', 'Final output', and 'Tap and Final output' being fed back to the input of the delay. the volume compensation is done using a 1/n multiplier, so that the final multipliers multiply by 1 for single inputs, or by half for two inputs. The fading is done by the 'p dip_mr' objects, whilst the gate~ objects do the switching, and the 'pipe' objects just delay th switching so that it happens when the volume is zero. But the really interesting stuff is in the 'dip' object:


Unfortunately, once you have seen it, then it isn't quite a magical as you might have expected. The line falls to zero in 50 milliseconds (ms), then stays there for 10 ms, then rises back to 1 in 50 ms again. The 0to 1 values of this 'fat' or 'dip' envelope are the multiplier value used in the multipliers, so there's no complicated conversions required. Here's the same thing expressed as a timing diagram:


So the trigger happens when one of the 'Freeze' buttons is clicked. The fade envelope starts to fall, and when it reaches zero, then the multiplier is zero, which means that no audio gets through the multiplier. The switch control has been delayed from when the trigger happened, and the audio signal is switched whilst the envelope is at zero and there is no audio signal getting through. Once the switching has happened, then the fade envelope returns back to 1, and the audio has been switched without any click happening.

This 'dip' fade envelope technique can be used whenever you want to switch from one audio signal to another without having a click. There are other ways to do it, including some that don't have any 'dip' in the audio, but this is a simple starting point for further explorations.

Filtering

I have struggled with the filtering all the way through the two 'AUDpiPOde' devices. In this 'A' version, I have revised the filtering again, so there is now only a single low-pass filter instead of two in cascade, and the high-pass filter is now a State Variable design because the 'subtracted low-pass' technique didn't seem to work very well (but then inside a feedback loop is always a bad place for any filter!). This is far from a perfect design, and full credit to the Ableton coders who have a far superior filter in the original Ping Pong Delay, and in the new 10.1 Delay devices. If only such a filter was available in MaxForLive...

Getting  AUDpiPOde-A 0v05

You can download AUDpiPOde-A 0v0for free from MaxForLive.com.

Here are the instructions for what to do with the .amxd file that you download from MaxforLive.com:

     https://synthesizerwriter.blogspot.co.uk/2017/12/where-do-i-put-downloaded-amxd.html

(In Live 10, you can also just double-click on the .amxd file, but this puts the device in the same folder as all of the factory devices...)

Oh, yes, and sometimes last-minute fixes do get added, which is why sometimes the blog post is behind the version number of MaxForLive.com...

Version 0.05 gives some idea of the development problems that I have had with these 'ping pong delay' devices! It seems that having ping pong delay plus several freeze modes is a good way to get confused about single routing, and I have made more mistakes than I want to think about. So my normal 'Work In Progress' label definitely applies to this device!

Modular Equivalents

In terms of basic modular equivalents, then AUDpiPOde-A would require two band-pass filters, two delays, some utility switches, and two mixers, giving a total of about 5 ME.






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