Phase vocoders

[dave2010]

Not quite sure where to put this (someone will no doubt tell me ...) as not quite hi-fi, not quite music. Guitarists (certainly the electronic variety) frequently use gadgets for effects, including devices such as phase vocoders, which can be used to change pitch amongst other things in real time. It is possible to do some of these effects with software plug-ins with some DAWs.

Are there people reading round here who actually use any of this stuff? Also, are most of the vocoders used for guitars, or are they used in a wider community.

Lastly, are there any recommended hardware units? [Go on, you can mention software as well, if you like!]

[mikeyb]

Are they like these?

https://youtu.be/QKDH5nqWujs

Just kidding, but every time I hear the word vocoder I always think of these characters from Chewin' the Fat

[Haselsh1]

My only connection here is that back in 1999 - 2002 I used the vocoder built in to my Zoom RFX-1000 unit whilst gigging. Loved it, very techno.

[Barry]

If they produce the same effect as a 'Voicebox' (made 'famous' by Peter Frampton), then I have to say I absolutely loath the nasal effect it produces. Its use has become fashionable with a lot of African pop music - I simply cannot listen to it!

[Macca]

Used on vocals sometimes, Cher is usually credited with re-introducing it with 'Believe' https://www.bing.com/search?q=cher+b...FORM=QBLH&sp=3

But Neil Young had also used it on vocals for 'Trans' some years prior. No video for that as it's terrible. 'I don't know what the fuck he thought he was doing' as a mate once remarked of it.

[Light Dependant Resistor]

Audacity has a vocoder as a plugin, a tutorial here gives instructions https://www.youtube.com/watch?v=yZPd5Xd4Wa8

King Crimsons 21st Century Schizoid Man featured it, particularly the version on "Earthbound" distorts the vocals
in a really unique way- well before any software , and was recorded live in February 1972

[dave2010]

OK - some interesting responses so far. I agree that some vocoder effects can be dreadful - though chacun ...

I have been particularly interested in so-called phase vocoders, which it seems can do pitch shifting without changing the timing. How this works is quite ingenious, though will almost certainly not be a "perfect" process - there will be artefacts which may be unwanted, or alternatively they may be considered minor, or acceptable. Sometimes users actually like artificial distortion and artefacts.

https://en.wikipedia.org/wiki/Phase_vocoder

My particular interest was in shifting the pitch of an instrument - downwards - to provide a bass part for practice purposes. I figured that I could make a recording of the bass part using a significantly higher pitched instrument, then push the pitch down. I was surprised that it actually worked, and didn't sound too bad, both with a one octave and a two octave adjustment. However, trying to do this up and down thing too often does definitely lead to problems. This approach worked to the extent that the phrasing in the original performance was clearly audible.

I used Audacity to do the pitch shifting - but of course that was in "off-line" mode - I had to record the musical part then apply the effect and then replay it.

There are plug-ins for some DAWs which will do similar things - possibly in real time for live events.

I wondered if there are any vocoders capable of working in real time, so that they could be used in live performances. Apparently there are some which are good enough for this, and there are musicians who are using such phase vocoders (either hardware or software based) for various purposes like that.

I'm interested to know if anyone has experience of such devices - but even off-line use would be of interest.

[dave2010]

I realise that the main thread title might send people slightly off track, so I thought I'd revisit this. It is true that vocoders can do horrible things to the sound of human voices. I started this investigation actually with trying to find out about pitch shifting. As noted, Audacity has a number of plugin effects - which include a vocoder effect, but also pitch shifters. I tried the pitch shifter in Audacity on some tracks I made up specially, and I was surprised that the pitch changed - as required - but the overall timing remained the same, and the aural qualities were largely retained.

Obviously one way of doing pitch shifting is to change the time base of a digital representation, either stretching or compressing the digital fragment, but to do a pitch shift without changing the length of the fragment - something has to give, but what?

Apparently the technology to do this is (often?/largely?) based on phase vocoders, though perhaps not all phase vocoders are the same - some may use different algorithms.

Algorithms in software, such as in tools like Audacity can be more or less complex. One might expect that with off line processing - spending more time on the execution of the algorithms - better results could be obtained than using cruder algorithms. Indeed, I think if the pitch shift effect in Audacity is chosen, there are options to do better quality outputs with longer processing time.

The problem with doing pitch shifting like this is that although the results may be rather good, there is too long a delay between the original recording and the pitch shifted version. So if a musician wanted to make a multi-track recording using pitch shifts as a method of simulating different instruments, it would require repeated "takes" and edits - though eventually the results might be quite good. Musicians who want to use live sounds could still do this by having pre-recorded fragments for playing during live performances.

I wondered if any pitch shift devices could work fast enough for "real time" use in performance, and it seems that there are hardware pitch shifters which electric guitarists use. I don't know how these work, or even if they have wide applicability.
In order to work well within a group context internal delays should probably be less than 20 milliseconds. Also, unlike the off-line pitch shifters, some compromses might be acceptable - the trade off being between better perceived "quality" versus responsiveness.

Players of electric guitars may be used to adding in so-called pedal (**) units, which can be used to modify the sounds produced, and these are capable of working in real time - (**) though not all actually have a pedal - more like an on/off switch. Some units, such as the Whammy - do have a pedal, which can be used to control the effects in a more modulated way.

My investigations to date suggest that these devices can work, but need to be matched carefully to the intended application. Used with electric guitars possibilities for unwanted feedback are minimised, though some users have clearly tried using these with acoustic guitars - which gives a slight increase in the risk of problematic feedback loops.

[Haselsh1]

Frampton's Talkbox was not the same as a vocoder and though I may well be wrong, I thought Cher's effort was autotune from within Pro Tools..?

[Haselsh1]

Mister Blue Sky though was an analogue vocoder.