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Practical use of 75% duty cycle?
http://forums.nesdev.com/viewtopic.php?f=6&t=16055
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Author:  FrankenGraphics [ Fri Jun 09, 2017 5:08 am ]
Post subject:  Practical use of 75% duty cycle?

Try as might, i've yet to find practical use of the 75%positive/25% negative duty. Theoretically, a fast music engine or support chip could perhaps toggle between 25% pos/neg at rates faster than the frequency of the tone to allow for more nuances in timbre or even PWM sweeps, that's all i can think of (but if that could be achieved, we could also AM/FM the pulse channels). But am i missing something that might matter when composing?

Author:  za909 [ Fri Jun 09, 2017 9:53 am ]
Post subject:  Re: Practical use of 75% duty cycle?

There is a practical use if you want phasing effects to behave differently. For example playing the same note, but with slightly offset pitch on two channels using 25% and 75% pulse-width results in a high-pass filter-sort of PWM effect instead of the usual phasing (it's recommended to retrigger the phase on purpose when you start doing this, however, so that you can get predictable results)

Author:  rainwarrior [ Fri Jun 09, 2017 10:19 am ]
Post subject:  Re: Practical use of 75% duty cycle?

In general it does not have much obvious use.

Unlike the other duty settings, that one starts high rather than at 0, so if you keep its frequency low enough and continually reset phase, you can use it as a crude 4-bit PCM via its volume control. Not sure why you'd want to when there's a perfectly good 7-bit PCM already, though.

The ability to make 2 squares destructively cancel each other by putting them in phase an at 25% and 75% is also interesting. Aside from the limited musical effect this could have by carefully detuning one for short periods of time, you could also use it for timing tests via "interferometry", etc.

I've seen people use a duty macro that flips between 25% and 75% each frame for a sort of 60hz buzz + 25% tone instrument. Kind of an ugly sound for most purposes, but it is at least kind of distinctive.

Author:  tepples [ Fri Jun 09, 2017 10:56 am ]
Post subject:  Re: Practical use of 75% duty cycle?

rainwarrior wrote:
so if you keep its frequency low enough and continually reset phase, you can use it as a crude 4-bit PCM via its volume control. Not sure why you'd want to when there's a perfectly good 7-bit PCM already, though.

Not interfering with the other sample that you're playing on $4010-$4013 channel.

Author:  FrankenGraphics [ Fri Jun 09, 2017 11:28 am ]
Post subject:  Re: Practical use of 75% duty cycle?

That's perhaps best reserved for short samples (perhaps percussives), and keep relatively long-ringing samples on the 7-bit side. Although.. 4-bit format will probably squeeze any punch out of the percussives

Author:  rainwarrior [ Fri Jun 09, 2017 11:39 am ]
Post subject:  Re: Practical use of 75% duty cycle?

tepples wrote:
rainwarrior wrote:
so if you keep its frequency low enough and continually reset phase, you can use it as a crude 4-bit PCM via its volume control. Not sure why you'd want to when there's a perfectly good 7-bit PCM already, though.

Not interfering with the other sample that you're playing on $4010-$4013 channel.

Again, not sure why you'd want to other than to demonstrate that you can, but yes technically you could play a DPCM sample while using a square PCM, as long as your comfortable with the DPCM fetches interfering with your square PCM in return? :P

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