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NB: This function is a newer version of STRETCH TIME.
infile – input analysis file
outfile – output analysis file
timestretch – stretch/shrink multiplier, which may itself vary over time. (Range: 0.0001 to 10000; <1 shrinks the time)
d-ratio – proportion of channels to discohere. (Range: 0 to 1)
d-rand – frequency randomisation of discohered channels. (Range: 0 to 1)
Timestretch stretches or shrinks the sound over time, without changing frequency. It creates extra time-windows to expand the overall time-base of the sound, without a change of frequency. The stretching itself can be time-varying. If the stretch factor TSTRETCH is <1, a time-shrinking occurs.
SPECTSTR is identical to the original STRETCH TIME process in respect of time-stretching, but adds the DISCOHERE and RANDOMISE parameters. It is not clear yet what is meant by "discohere" in the context of time-stretching.
See STRETCH TIME
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1 Stretch above the frq_divide
2 Stretch below the frq_divide
infile – input analysis file made with PVOC
outfile – output analysis file
frq_divide is the frequency above or below which spectral stretching takes place
maxstretch is the transposition ratio of the topmost spectral components
exponent specifies the type of stretching required (> 0)
-ddepth stretch effect on source (from 0 (no effect) to 1 (full effect)
depth can vary over time
The STRETCH SPECTRUM process is a means to warp the spectrum of a sound. For example, a harmonic sound may become inharmonic, in a controllable way. (It is derived from CDP's former function SPECE.)
The STRETCH SPECTRUM process gives us control over the way in which we stretch the spectrum of a sound. It should be used for stretching as opposed to shifting the spectrum of a sound. The stretching operation begins at the frq_divide and proceeds upwards.
The main thing to appreciate regarding STRETCH SPECTRUM is that the stretch factor is scaled across the frequencies, beginning at 1 at the frq_divide (no stretch) and gradually increasing or decreasing until reaching maxstretch of the uppermost channel in which a frequency is found (not > Nyquist). Overall, it is rather like stretching or compressing a rubber sheet – the whole sheet is warped by the stretching process and the partials are thereby moved.
The scaling of maxstretch is linear (even increments) when exponent is an integer. If exponent is < 1.0, the stretch curve will rise quickly and then tend to even out. If exponent is > 1.0, the stretch curve will start to rise slowly and then gradually rise more quickly.
The stretching operation expands or contracts the frequency gap between partials. The Technical Discussion offers some supplementary observations about how to think about what this means.
Under this operation the frequency ratios between the partials of the original sound are not preserved. Thus, for example, a sound with harmonically related partials, e.g., 100, 200, 300, 400 Hz (common factor, 100) will end up with partials which are not harmonically related, e.g., 110, 213, 316, 419 Hz (no common factor). Such an inharmonic spectrum will normally be perceived as containing several pitches or (with a small stretch) strong spectral colouration. So the stretching operation usually increases inharmonicity and the timbral richness of a sound.
Use of maxstretch
Maxstretch specifies the amount of stretch to be applied to the partial found in the uppermost channel. The partial in the channel corresponding to frq_divide does not move (maxstretch = 1). Inbetween components will be moved by an amount corresponding to their relative position.
Special Note - components stretched beyond the Nyquist frequency (sr/2) are made to disappear so that the maximum component we will 'hear' will be the one marked 'X' in the above diagram. Hence a maxstretch value > 1 may be something of a fiction in practice because partials stretched beyond Nyquist actually disappear.
The image of frequencies on a rubber sheet, referred to above, can help point us in the direction of the possible applications of this process. We can picture the frequencies drawn on the sheet with the lower frequencies at the bottom and the higher frequencies at the top. When we stretch or compress the bottom of the sheet, the frequencies get further apart (i.e., pushing the bottom frequencies lower) or closer together (higher). This will deepen or tighten the tone, as well as make it become more inharmonic at the same time.
Similarly, when the top of the sheet is stretched or compressed, the frequencies become further apart (higher: top ones pushed up) or closer together (this will lower and bunch up the high frequencies), with a similar increase in inharmonicity.
As ever, the results are greatly affected by the nature of the infile. Experiments with a vocal infile should reveal the way it acts on a sound in an easily perceptible manner. Probably the single most important parameter is frq-divide: where the split point is placed. The quickest way to determine the frequency range of a sound is to use a tuning fork. Trial and error (i.e., guessing) can also be a fairly quick way to get results. Charts of the frequency ranges occupied by various types of sound can also be instructive.
For further discussion of stretching frequency components see: Stretching in the Spectral Domain
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NB: A newer version of this process is available: SPECTSTR
In Mode 2, the program calculates the length of the output only, and does not produce an outfile.
infile – input analysis file made with PVOC
outfile – output analysis file
timestretch single value or the name of a time timestretch breakpoint file of values. Timestretch is used as a multiple.
The key here is to time stretch the sound without altering the pitch. This can be used either to lengthen or compress the duration. Formerly SPECSTR, meaning 'spectrum time-stretching', the internal prolongation of spectral data is itself subject to temporal control according to the amount of stretch specified at different times.
The fact that the time can be set in the breakpoint file provides an important tool. For example, it can be used to retain the ability to recognise the source of the sound. We often can identify a sound according to the way it begins, its 'attack transient'. This can be kept unchanged by giving a timestretch of 1 for this part of the sound. Then the stretching can begin, so we retain the recognisability of the sound while stretching it in time. Retaining recognisability is often a very important compositional issue.
The breakpoint file sets time and timestretch factors in such a way that change is gradual (interpolated) between different times with different stretch factors. For example:
[time stretch] 0.0 1.0 1.0 1.0 2.0 2.5During the 1st second of the sound, there will be no change. During the 2nd second of the sound, the spectrum will be gradually stretched to 2.5 times its original length.
Two different stretch factors cannot be assigned to the same time. At least a slight difference in time is required. So, for virtually instantaneous changes in the stretch factor, use values such as:
[time stretch] 2.0 2.5 2.01 1.5
The stretching can be imagined in a quite literal way. This is illustrated by time stretching a sound which contains strong beat patterns. As the stretch gets longer, one really begins to hear the pulse slow down.
Very long time stretches might best be done in stages rather than all at once. If the whole sound is not required, one way to proceed is to cut out a portion after each time stretch (say a stretch factor between 5 and 10) with SPEC CUT and then restretch that portion. Otherwise, time is wasted as the sound gets longer and longer.
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Example command lines (in these examples 'bdt.wav' is 4.9 seconds long):
Mode 1. -c flag:
(1)stretcha -c -d8 -T90[Result: 12 beats (8 seconds fills 12 beats at MM = 90]
(2)stretcha -c -fbdt.wav -T108[Result: 8.819878 beats (the input soundfile fills 8.819878 beats at MM=108]
(3)stretcha -c -b8 -t60 -T72[Result: 9.6 beats (8 beats at MM=60 becomes 9.6 beats at MM=72]
Mode 2. -s flag:
(1)stretcha -s -d8 -D12[Result: stretchfactor = 1.5 (use to make 8 seconds fill 12 seconds]
(2)stretcha -s -d8 -B4 -T50[Result: stretchfactor = 0.6 (use to make a specified duration fill 4 beats at MM=50]
(3)stretcha -s -fbdt.wav -D12[Result: stretchfactor = 2.449014 (use to make a 4.9 second soundfile fill 12 seconds]
(4)stretcha -s -fbdt.wav -B4 -T50[Result: stretchfactor = 0.977605 (use to make a 4.9 second soundfile fill 4 beats at MM=50]
(5)stretcha -s -b8 -t84 -D12[Result: stretchfactor = 2.449014 (use to make 8 beats at MM=84 fill 12 seconds]
(6)stretcha -s -b8 -t84 -B6 -T96[Result: stretchfactor = 0.5625 (use to make 8 beats at MM=72 fill 6 beats at MM=96]
Mode 3. -s flag:
stretcha -s -t66 -T120[Result: stretchfactor = 0.550 (use to convert a passage moving at tempo MM=66 to one moving at tempo MM=120]
Mode 1: (three input options ⇒ a specified tempo)
-c – takes as inputs: a duration OR a soundfile OR (a number of (possibly fractional) beats and a tempo) and also an output tempo
1. IN: -ddur OUT: -Touttempo ⇒ displays how many beats there will be at the new tempoMode 2: (three input options ⇒ to a specified duration OR to a number of beats at a given tempo)
2. IN: -fsndfile OUT: -Touttempo ⇒ displays how many beats the length of the input soundfile will fill at the new tempo
3. IN: -bbeats -ttempo OUT: -Touttempo ⇒ displays how many beats there are at the new tempo
-s – takes as inputs: a duration OR a soundfile OR a number of (possibly fractional) beats and a tempo and also an output duration OR a number of (possibly fractional) output beats and an output tempo
1. IN: -ddur OUT: -Doutdur ⇒ displays the stretchfactor needed to change one duration to the otherMode 3: (input tempo ⇒ output tempo)
2. IN: -ddur OUT: -Bbeats -Ttempo ⇒ displays the stretchfactor needed to convert an input duration to a given number of beats at a given tempo
3. IN: -fsndfile OUT: -Toutdur ⇒ displays the stretchfactor needed to convert a given input soundfile to a specified duration
4. IN: -fsndfile OUT: -Bbeats -Ttempo ⇒ displays the stretchfactor needed to convert a given input soundfile to a given number of beats at a given tempo
5. IN: -bbeats -ttempo OUT: -Toutdur ⇒ displays the stretchfactor needed to convert a given number of beats at a given tempo to a specified duration
6. IN: -bbeats -btempo OUT: -Touttempo ⇒ displays the stretchfactor needed to convert a given number of beats at a given tempo to specified number of beats a specified tempo
-s – takes as inputs: an input tempo and an output tempo;1. IN: -ttempo OUT: -Ttempo (displays the stretchfactor to change from the first tempo to the second)
dur – an input length of time in seconds
sndfile – an input soundfile
beats – an input number of (possibly fractional) beats
tempo – an input tempo at which the beats occur
outdur – an output length of time in seconds
outbeats – an output number of (possibly fractional) beats
outtempo – an output tempo
STRETCHA is a utility program for use with STRETCH TIME. It takes a bit of calculating to work out how to make a soundfile fit specific numbers of possibly fractional beats. STRETCHA does the work for you. When a soundfile is named as in input, its duration is automatically determined. It can also handle durations and beats as inputs or as outputs to add extra flexibility.
Note that, if you do not have the environment variable CDP_SOUND_EXT SET, you will have to include the .wav extension with the soundfile name. The commandline environment also enables you to write the output to a file rather than just see it displayed on the screen. This is done with the redirection symbol > as in this example command line:
stretcha -s -fbdt.wav -D8 >strfac.txt.The stretch factor is then used with the CDP program STRETCH TIME. While most of the options are fairly straightforward, note that some possibilities are not covered directly but require two steps. The three -c options of Mode 1, for example, which convert to a specified tempo, display only "number of beats at new tempo" – it does not actually give a stretchfactor. It gives the number of beats that will result. You can then use this information to fill in the 'beats' parameter value in Mode 2 options to get the stretchfactor to convert the input beats and tempo to a specified duration (option 5) OR tempo (option 6).
To illustrate this two step process, if you input a duration of 4 seconds and an outtempo of MM = 90: Mode 1 option 1 (stretcha -c -d4 -T90), the program tells you that 6 crotchets will result. Now you can use that information to calculate the required stretchfactor to fill a specified duration: Mode 2, option 5 (stretcha -s -b6 -t90 -D10), yielding a stretch factor of 2.5. Thus 4 seconds changes to 6 crotchets at MM = 90, and this will fill 10 seconds with a stretch factor of 2.5. You have thus gone from duration to duration while specifying a specific number of beats at a given tempo. What you need to do depends on the nature of the material with which you start.It may also be useful to know the actual duration that results from a given number of beats at a given tempo. For example, if you want 4 beats at MM = 72 to be changed to 7.5 beats at MM = 108, you will have to know what duration the later produces (to use Mode 2 option 5). The program doesn't provide this facility, but it is easily calculated as
numbeats * 60.0/tempo, which in this case is7.5 * (60.0/108), giving 4.1666 seconds. Then you can use:stretcha -s -b4 -t72 -D4.1666, giving a stretch factor of 1.249980.You can also calculate the duration of the input portion of beats at a tempo and then use the input and output duration option (Mode 2 option 1). Thus
4 * (60.0/72) = 3.3333:stretcha -s -d3.3333 -D4.1666gives a stretch factor of 1.249980.
It may be helpful to separate out all the options. This reveals how easy it is to use. In the examples, the soundfile bdt.wav is used. Its duration is 4.90 seconds, as shown bydirsf bdt.wav. I've used numbers that enable you to see and compare the results quite easily.Mode 1:
1. Inputs: duration and output tempo, Output: number of beats
2. Inputs: soundfile and output tempo, Output: number of beats
stretcha -c -ddur -Touttempo
Example:stretcha -c -d4 -T120
Result: number of beats at new tempo = 8.0 crotchets
3. Inputs: beats & tempo and output tempo, Output: number of beats
stretcha -c -finsnd.wav -Touttempo
Example:stretcha -c -fbdt.wav -T120
Result: number of beats at new tempo = 9.799864 crotchets
stretcha -c -bbeats -ttempo -Touttempo
Example:stretcha -c -b4 -t60 -T120
Result: number of beats at new tempo = 8.0 crotchets
Mode 2:
1. Inputs: duration and output duration, Output: stretch factor
2. Inputs: duration and output beats & tempo, Output: stretch factor
stretcha -s -ddur -Doutdur
Example:stretcha -s -d4 -D8
Result: stretch factor is 2.0
3. Inputs: soundfile and output duration, Output: stretch factor
stretcha -s -dduration -Bbeats -Ttempo
Example:stretcha -s -d4 -B8 -T90
Result: stretch factor is 1.333333
4. Inputs: soundfile and output beats and tempo, Output: stretch factor
stretcha -s -finsnd.wav -Doutdur
Example:stretcha -s -fbdt.wav -D8
Result: stretch factor is 1.632676
5. Inputs: beats & tempo and output duration, Output: stretch factor
stretcha -s -finsnd.wav -Boutbeats -Touttempo
Example:stretcha -s -fbdt.wav -B8 -T90
Result: stretch factor is 1.08845
6. Inputs: beats & tempo and output beats & tempo, Output: stretch factor
stretcha -s -bbeats -ttempo -Doutdur
Example:stretcha -s -b4 -t60 -D8
Result: stretch factor is 2.0
stretcha -s -bbeats -ttempo -Boutbeats -Touttempo
Example:stretcha -s -b4 -t60 -B8 -T90
Result: stretch factor is 1.333333
Mode 3:
1. Input: tempo, Output: tempo
stretcha -s -ttempo -Touttempo
Example:stretcha -s -d60 -D90
Result: stretch factor is 0.666667
I (AE) asked Trevor to write this program several years ago in order to help create passages of music using regular beat patterns. The source code has been lost for all these years, but an enquiry from Arthur Green, who found the program in his directory of CDP programs and discovered that it would not work on a 64-bit PC, led Richard Dobson to search out the source and recompile it for us.
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To help visualise what might be happening with the spectrum, here are some observations about 'interval', 'frequency' and 'partials':
The stretching operation therefore expands or compresses the frequency (i.e., partial) spacings. But in the higher end of the spectrum, it is useful to remember that the intervals involved will be very small. The aural effect of the stretching process will relate in part to the inharmonic relationships created (or altered) among the intervals defined by the frequencies.Bear in mind that the partials of a tone are closer together in pitch the higher their frequency. This is because pitches are the logarithm of frequency. This means that going up in pitch by equal amounts, we add intervals – such as going up by one octave – but we multiply frequencies – times 2 to go up one octave.
For example, going up one octave from 400 Hz takes us to 800 Hz, but going up an equivalent one octave from 4000 Hz takes us to 8000 Hz, i.e., 10 times as far in terms of frequency.
Looking at this the other way round, going up by 400 Hz, e.g., from 400 Hz to 800 Hz, takes us up an octave; but going up 400 Hz from 4000 Hz to 4400 Hz takes us up only a very small interval.
The harmonic partials, being integer multiples of the fundamental, are equally spaced in frequency (e.g., 400 (400 * 1), 800 (400 * 2), 1200 (400 * 3), 1600 (400 * 4) – all are 400 Hz apart). As a result, the pitch-intervals between them get smaller and smaller as we rise in frequency.
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