Rise Time

[cardong]

What is the default rise time in Audacity for generating a pure tone? How could one change this?

[kozikowski]

Since Audacity is a digital tool, I think it's instantaneous. I've run into problems where I get a serious click because I started a sine wave too quickly.

You can use the Fade In and Fade Out tools to smooth the transition points. You calculate the fade time versus the frequency. Figure a Fade In is 1/4 of a modulating sine wave, so four times that time and invert to get the frequency. The sum of the two tones needs to be too low to hear.

You can't ever reduce problems to zero because any change to a sine wave produces harmonics.

The basic principal of AM broadcasting.

Koz

[whomper]

Since Audacity is a digital tool, I think it's instantaneous. I've run into problems where I get a serious click because I started a sine wave too quickly.

Koz

rise time is not slew rate
digital can slew "infinitely" fast
but the rise time should depend on the freq of the sin wav you are trying to do -- assuming you start its rise at a zero crossing point
and of course a square wave will cause a click with a sudden jump
because the very high freqs in the fourier equivalent will permit arbitrarily fast rise times

[kozikowski]

<<<rise time is not slew rate>>>

No, it's not, but we're talking about the tone rise time, not the waveform rise time.

<<<digital can slew "infinitely" fast>>>

Tones can also rise infinitely fast. I just tried it and Generate can start a tone at maximum amplitude from zero. This will create a serious click at the start point if I left it and tried to use it that way.

<<<assuming you start its rise at a zero crossing point >>>

I don't think Generate has any other option.

<<<and of course a square wave will cause a click with a sudden jump
because the very high freqs in the fourier equivalent will permit arbitrarily fast rise times>>>

There is no tool to create square waves I know of. Actual slew rate problems will turn a sine wave into a triangle wave, not square.

This is a straight modulation problem. If you can keep the modulation products below 20 Hz, than you should get a tone that turns on without clicking.

Koz

[steve]

<<<assuming you start its rise at a zero crossing point >>>
I don't think Generate has any other option.

The "Generate menu > Tone..." does not have any other option, but if you use it across multiple tracks it does not start each track from zero.
This tone will start on the first track at zero, but when it gets to the end of the track and starts to generate on the second track it will simply continue from whatever point in the phase cycle it has reached.

Here's an example of creating a sine tone over a very short selection period (a little over 3/4 of a cycle) across two tracks. You can see how the waveform has reached approximately 300 degrees at the end of the selection in track 1, then continues on track 2 from the 300 degree phase point.

2 track sine tone

tracks000.png (10.16 KiB) Viewed 1647 times

There is no tool to create square waves I know of.

Generate menu > Tone > Waveform (drop-down selection box) > Sine/Square/Sawtooth/Square,no alias.

The "Square, no alias" option will produce a bandwidth limited square wave (equivalent to the sum of a large but finite number of odd harmonics), whereas the "Square" option will produce a square wave with "infinitely" fast rise/fall time (rise time = 1/sample rate).

This is a straight modulation problem. If you can keep the modulation products below 20 Hz, than you should get a tone that turns on without clicking.

Absolutely.
By way of example, if you generate a sine tone and delete a couple of cycles from the middle - starting and ending at zero crossing points - you will hear a "click" during playback where the "gap" occurs. It is, as you say, not the "waveform rise time" that causes the click, but the "tone rise time". Or to put it another way, it is the rate of change that causes the click. Introducing a short fade time will eliminate the click.

[whomper]

<<<rise time is not slew rate>>>

No, it's not, but we're talking about the tone rise time, not the waveform rise time.
Koz

Live and learn. what is *tone* rise time? how does that differ from those others? how can anything that represents a pure sine wave rise faster than that sin's fundamental freq?

[steve]

During a constant sine tone, the rate of change is constant. Not so when the waveform starts from DC.

Consider this waveform:

440Hz Sine

firsttrack000.png (8.81 KiB) Viewed 1628 times

This is a 440Hz sine wave in which every other cycle has been removed.
What will the spectrum look like?

[whomper]

thanks
i see the issue and the problem/difference
would not have called it by that name if i had done it
but if that is what it is known by then now i know what it is