Controls: Gen Dlg >> Stream >> Wave >> Band >> Rise/Fall
Macros: BandRise, BandFall

To create a band of noise between two frequencies, set Rise Fc to the lower frequency and Fall Fc to the upper frequency.

To create a wideband noise with a gap between two frequencies, set Fall Fc to the lower frequency and Rise Fc to the upper.

Like all other frequency controls in Daqarta, these respond to the various Frequency entry modes. You can set the current (global) mode via the dialog pop-up button below the Rise Fc control.

See Ratios and Octaves if you want to set the band limits such that they define an octave or fractional octave about a specified center frequency, such as "1/3 octave centered at 10 kHz".

Although the Rise and Fall controls set the "frequency" of a noise band, this is not a value that can be modulated directly. Just like the other Noise sources, when Band is active, the modulator options are reduced to Burst and AM only... no FM, Phase, or Sweep. However, it is indeed possible to create a swept or modulated Band center frequency under some conditions.

This technique works best for narrow to moderate width noise bands. Set Rise to 0 and Fall to half the desired bandwidth, giving a low-pass noise. Now go to a higher stream and set up a sine wave whose frequency will be the center of the noise band. Set the AM Source on that stream to the stream with the low-pass noise, and set AM Depth to 200%. (That changes the AM to straight multiplication.) Turn on AM and you will see the low-pass noise band mirrored about the selected center frequency.

To understand what's happening here, consider that when two frequencies are multiplied together, the product contains only the sum and difference of the original frequencies. (Check out the AM Depth section for the formula.) That applies to the individual low-pass noise frequencies as well, so each instantaneous frequency in the original noise becomes two frequencies in the product, one above and one below the center frequency.

Now all you need to do is apply a Frequency Sweep or FM to the center frequency, and it becomes applied to the noise band instead. Note that if you use too wide a band, or sweep too far, you can run the noise above the Nyquist frequency or below zero. In those cases the band folds back on itself, which might not give the desired result.

Astute observers might object that a noise band created via this mirror technique is not really the same as the equivalent band created in the normal way. The fact that the noise components always come in matched pairs means it is not totally random. But unless the band is extremely narrow, such that there are not a lot of other components, this fact will not be detected via listening tests. This technique was used for years with real electronic filters, oscillators, and analog multipliers to create adjustable narrow noise bands for psychoacoustics tests, long before the digital era.

This approach also works for noise gaps, but is more limited. Here you use exactly the same concept, but instead of low-pass you use a noise band. The basic idea is that when you multiply this by the center frequency, you will have two bands flanking the center. The distance from the center frequency to each band will be equal to the Rise frequency. (Remember, the lower band has been mirrored; it starts from the center frequency and runs downward.)

Now the trick is to adjust the Fall frequency, which controls the outer band edges, so that there are no extra gaps at the extreme high and low frequencies. If the gap were to be at a fixed frequency in the center of the range, this would clearly be no problem; you'd just set Fall to half the total Nyquist range and the outer edges would fall neatly at the ends.

But if Fall is larger than half, the outer edges will wrap back into the main range, raising the level of the wrapped region by 3 dB. This is one drawback of this method, but for many applications the slight level change may not be a problem.

Next you have to determine how far you will need to move your noise gap from the center of the range. The farther down you move it, the more wrap you get at the bottom and the more chance for another gap to slip in from the top. You need to make Fall large enough so that when the gap has moved to its lowest extreme, there will still be no gap at the high end. But if you make it too large, the wrapped portion will fall into the desired gap from the bottom. The exact same consideration applies when moving the gap up instead of down.

This method thus turns out to be best for modest gaps with modest motion near the center of the range. It's cumbersome to set up, but nevertheless this does provide a way to get a moving gap.

Macro Notes:

L.1.BandRise=1k sets Left Stream 1 Rise to 1000 Hz. L.1.BandFall=4k sets Fall to 4000 Hz. The actual values may be quantized depending upon the setting of the Frequency Entry Step Mode

To increment or decrement the current Rise value, use BandRise=>1 or BandRise=>-1, respectively. The increment size is set via the Frequency Entry Step Mode. Only +/-1 step is accepted.

See also Band-Limited Noise, Noise Waves, Wave Dialog.

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