Subject: Re: [linux-audio-dev] LADSPA 64bit FP support ?
From: James McCartney (asynth_AT_io.com)
Date: Wed Mar 22 2000 - 08:19:03 EET
on 3/21/00 11:01 PM, Paul Barton-Davis at pbd_AT_Op.Net wrote:
>> I'd like to see a real world example where you have a signal that is full 24
>> bits peak to peak and you can hear some distortion in the low 4 bits.
>> Because other than full scale peak to peak signals you will not lose the
>> lower bits using 32 bit FP.
>
> Very true, but sadly irrelevant. As Bill Gribble put it to me: the
> bits-in == bits-out test is fundamental. If you can't record a digital
> data stream and reproduce it bit-for-bit, then you're in trouble from
> the start. a 32 bit float is in extreme danger of failing to do this
> if there are any attempts at symmetric transforms on 24 bit integer
> data that require use of the exponent in an intermediate value. Given
> the tendency of engineers to want to do tracking with a hot signal, it
> doesn't take much to get a 24 bit signal to the point where mixing,
> say, 10 of them together gets a value that when doubled, exceeds the
> capacity of the mantissa. When you halve it again, there's a good
> chance that bits-in != bits-out.
(Doubling and halving have no effect on the mantissa at all...?)
I still think that the only reason we have a 24 bit integer is to overcome
the limitations in SNR of integer signals at low amplitude. So lower order
bits on full scale signals are irrelevant. They can be lost with no loss of
quality. It is the very quiet signals in the 24 bit integer format that
cause the loss of quality. and that happens before you get into FP.
Why is bits-in == bits-out so important?
We are talking about losing bits that are over 120 dB down. No one is ever
going to hear it. You have to be in a very quiet place to hear even -60 dB.
The only reason those bits need to be there for integer signals is to
represent the quiet signals. FP doesn't have a loss of quality problem for
quiet signals.
--- james mccartney james_AT_audiosynth.com <http://www.audiosynth.com>
SuperCollider - a real time synthesis programming language for the PowerMac.
<ftp://www.audiosynth.com/pub/updates/SC2.2.7.sea.hqx>
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