>- convert the 24bit input values (from the ADC) to 64bit double precision floats
>- do all computations using the double format (because the speed loss over
>using 32bit floats is not that big as you might think)
>- at output stage truncate/round the 24 upper bits of the 64bit double to a
>24bit word (integer) and send it to the DAC.
>If that moves an int to/from float conversions to hardware, it might
be
>a good idea. I'm not sure how expensive the conversions are with
respect
>to the main DSP stuff: if it is only 1% of all processing, then
we might
>want to have or not to have the conversions in hardware.
OK, this is the kind of info that I am looking for. Benno suggests converting the 24 bit integers from the adc to float within the computer. Juhana recognizes some benefit to converting to float outside the computer but doesn't know how much overhead is associated with converting inside the computer. This allows me to ask more detailed questions. Also consider this. On the dac side of the issue, a trend that has already started is that of transmitting digitally to a self powered monitor with a built in dsp crossover that outputs to dac's that are built into the input stage of the power amps. With a system like mLAN there are so many channels available that one could do the crossover dsp within the computer then multiplex the filtered signals and send them to the monitor. The monitor would only need a demultiplexer to feed the individual dac's, not dedicated dsp. Economics suggests that this technique is inevitable. This would also be great because one could fine tune the crossover for a given installation using a gui, and with some high quality mic's at the listening position, automation is possible. This brings up the issue of low pass, band pass, and high pass dsp filters.
So here are some detailed questions. These questions relate to defining an audio production environment where audio is converted to digital as close to the source as possible, and is kept digital as long as possible before being converted to sound. Also the digital resolution is higher than the targeted consumer format so that low level rounding/ truncating errors are eliminated in the final bit decimation process.
1) In a digital mixing environment, where everything is a plug-in, is it preferred to convert the audio to float before performing dsp always, most of the time, occasionally, or never? Benno suggests converting to 64 bit double precision float as policy.
2) When converting audio samples to float, either from the soundcard or from hd, is the overhead associated with this conversion high enough that storing audio to disk as float or delivering audio from the soundcard as float would provide a meaningful performance gain, since conversion immediately prior to dsp would no longer be necessary? Also consider the overhead of converting back to integer for final output, since this can also be eliminated by using float dac's.
3) Are the currently superior low pass, band pass, and high pass filters using float or integer math?
>What is a high quality 24-bit DAC? Or ADC? Last I checked the 24-bit
only
>means the width of the data port, not the quality.
The best adc that I have read about still swamps the lower 4 bits with noise. DAC's are a different beast and I don't know where they are right now.
Tom
