| Author |
Message |
john
Guest
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 Posted:
Sun Dec 18, 2005 5:15 pm Post subject:
Audio processing -- fixed or floating point? |
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I just watched a video on Microsoft's Channel 9 site about the new
audio processing code that they are putting in Windows Vista (the next
version):
http://channel9.msdn.com/Showpost.aspx?postid=145665
They are changing from an "int16 pipeline" to a "float32 pipeline".
They assert that using floating point for audio processing yields far
better fidelity than int16, and also point out that on a Pentium,
floating point is efficient (the old int16 code dates back to when they
supported 486 chips with no floating point).
Not being an audio person, I was wondering what some of the experts
here would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
John |
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robert bristow-johnson
Guest
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| Posted:
Sun Dec 18, 2005 5:15 pm Post subject:
Re: Audio processing -- fixed or floating point? |
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the 32-bit float carries all of the information of a 16-bit int and
even more. of course it will sound better (at the expense of twice
the memory in the pipeling.
the issue that us audio guys argue about is given a particular word
width of N bits. which sounds better? an N bit float (how are N bits
divided into exponent and mantissa?) or fixed point?
r b-j |
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Andrew Reilly
Guest
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| Posted:
Mon Dec 19, 2005 1:15 am Post subject:
Re: Audio processing -- fixed or floating point? |
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On Sun, 18 Dec 2005 04:32:55 -0800, john wrote:
| Quote: | I just watched a video on Microsoft's Channel 9 site about the new
audio processing code that they are putting in Windows Vista (the next
version):
http://channel9.msdn.com/Showpost.aspx?postid=145665
They are changing from an "int16 pipeline" to a "float32 pipeline".
They assert that using floating point for audio processing yields far
better fidelity than int16, and also point out that on a Pentium,
floating point is efficient (the old int16 code dates back to when they
supported 486 chips with no floating point).
|
PCs pretty much since the P6 core (Pentium 2/3, M, 4) have had better
floating point performance than integer. There's whole extra chunks of
hardware to do that work, leaving the integer part to get on with sorting
out addresses and conditional code. Since the SIMD instruction sets were
introduced (3DNow!, SSE{,2,3}) floating point has just become
comparatively stronger. This is mostly in support of video games, where
smallish floating point numbers seem to be essential for geometry
computation. Audio gets a nice free ride as a result. When the SIMD
extensions first came out, the heavy number crunching guys complained,
because 32-bit floats aren't actually terribly useful for big maths
problems. Now (SSE3) there's SIMD double precision maths, so everyone's
happy.
So yes: efficient.
Better audio quality? Well, usually. that's actually a bit of a curly
question. When 16-bit integers are used in audio, the results of products
are usually accumulated in (integer) words that are 32 (or, preferably,
more) bits wide. That means that if you only use 32-bit floating point
numbers for your calculations, you might actually be losing seven or eight
LSBs that an integer computation would preserve (a 32-bit float has about
25 bits of mantissa). Are those bits significant, and do they contribute
to significant noise floors? Well, that's pretty much impossible to say in
general, but the noise analysis is much more complicated in floating point
than it is in integer, anyway. If you're going to be dithering down to 16
bits for final output chances are that will mask any introduced
computation noise. If you're going to output 24-bit digital data somehow,
then maybe you need to be careful with those floats. If you only have
32-bit floats, you might have more trouble building a very low frequency
IIR filter than someone who can use double-precision (or higher) fixed
point arithmetic.
| Quote: | Not being an audio person, I was wondering what some of the experts here
would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
|
Always? No, it depends on the situation. In a PC? Probably. In a
portable device? Probably not: floating point computation (particularly
the "IEEE-754 compliant" variety) requires more transistors and so
consumes more power than broadly equivalent integer computation. So for
now, most portable music devices use fixed point, and many wall-powered
devices use floating point, but some of these use fixed point too, perhaps
for cost reasons, perhaps for historical reasons.
FWIW Apple's "Audio Units" system in MacOS X is floating point, and
AltiVec is good at 32-bit floats (but not doubles).
Cheers,
--
Andrew |
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Vladimir Vassilevsky
Guest
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| Posted:
Mon Dec 19, 2005 1:15 am Post subject:
Re: Audio processing -- fixed or floating point? |
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john wrote:
| Quote: | I just watched a video on Microsoft's Channel 9 site about the new
audio processing code that they are putting in Windows Vista (the next
version):
http://channel9.msdn.com/Showpost.aspx?postid=145665
They are changing from an "int16 pipeline" to a "float32 pipeline".
They assert that using floating point for audio processing yields far
better fidelity than int16, and also point out that on a Pentium,
floating point is efficient (the old int16 code dates back to when they
supported 486 chips with no floating point).
Not being an audio person, I was wondering what some of the experts
here would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
|
Floating point greatly simplifies the development. Also the floating
point is indeed very efficient on the P5+ CPUs. They do not do much of
audio processing in Windows, so I doubt if there is going to be any
noticeable difference.
Technically speaking, the single precision IEEE-754 float accuracy may
not be sufficient for the certain aspects of audio processing. The
32-bit int may produce better results.
Specifically, the float point will cause problems with IIR filters:
limit cycle oscillations and noise.
The bottom line is: "The chicken laid an egg, but she cackles like she
laid a planet". I don't expect any great improvements from the use of
the floating point and I don't consider it as a big deal at all.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com |
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john
Guest
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| Posted:
Mon Dec 19, 2005 1:15 am Post subject:
Re: Audio processing -- fixed or floating point? |
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Vladimir Vassilevsky wrote:
| Quote: | john wrote:
I just watched a video on Microsoft's Channel 9 site about the new
audio processing code that they are putting in Windows Vista (the next
version):
http://channel9.msdn.com/Showpost.aspx?postid=145665
They are changing from an "int16 pipeline" to a "float32 pipeline".
They assert that using floating point for audio processing yields far
better fidelity than int16, and also point out that on a Pentium,
floating point is efficient (the old int16 code dates back to when they
supported 486 chips with no floating point).
Not being an audio person, I was wondering what some of the experts
here would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
Floating point greatly simplifies the development. Also the floating
point is indeed very efficient on the P5+ CPUs. They do not do much of
audio processing in Windows, so I doubt if there is going to be any
noticeable difference.
Technically speaking, the single precision IEEE-754 float accuracy may
not be sufficient for the certain aspects of audio processing. The
32-bit int may produce better results.
Specifically, the float point will cause problems with IIR filters:
limit cycle oscillations and noise.
The bottom line is: "The chicken laid an egg, but she cackles like she
laid a planet". I don't expect any great improvements from the use of
the floating point and I don't consider it as a big deal at all.
|
Interesting. In the video the MS guys leave the the impression that the
fidelity improvements will mainly be in situations where the legacy
integer solution was difficult to properly implement in the first
place.
John |
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Steve Underwood
Guest
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| Posted:
Mon Dec 19, 2005 9:15 am Post subject:
Re: Audio processing -- fixed or floating point? |
|
|
Vladimir Vassilevsky wrote:
| Quote: |
john wrote:
I just watched a video on Microsoft's Channel 9 site about the new
audio processing code that they are putting in Windows Vista (the next
version):
http://channel9.msdn.com/Showpost.aspx?postid=145665
They are changing from an "int16 pipeline" to a "float32 pipeline".
They assert that using floating point for audio processing yields far
better fidelity than int16, and also point out that on a Pentium,
floating point is efficient (the old int16 code dates back to when they
supported 486 chips with no floating point).
Not being an audio person, I was wondering what some of the experts
here would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
Floating point greatly simplifies the development. Also the floating
point is indeed very efficient on the P5+ CPUs. They do not do much of
audio processing in Windows, so I doubt if there is going to be any
noticeable difference.
Technically speaking, the single precision IEEE-754 float accuracy may
not be sufficient for the certain aspects of audio processing. The
32-bit int may produce better results.
Specifically, the float point will cause problems with IIR filters:
limit cycle oscillations and noise.
The bottom line is: "The chicken laid an egg, but she cackles like she
laid a planet". I don't expect any great improvements from the use of
the floating point and I don't consider it as a big deal at all.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
|
There is something of a tradeoff when using floating point for DSP on a
Pentium or Athlon. As everyone else has said, basically floating point
performance blows away integer performance on modern processors. As long
as you are careful about floating to integer conversion (use the C99
rint() function) the conversions can be quick and painless. However,
there is one snag with floats on Pentiums and Athlons - denormalisation
on underflow. When a float gets really close to zero, and will no longer
normalise properly, performance is really really terrible. The situation
is treated as an exception, and handled by complex exception processing.
People try nudging the numbers to avoid this. However, strict avoidance
requires comparisons and branching, which means possible pipeline
stalls. Its all very messy. Try Googling for something line "dsp float
denormalize pentium" and you should hit a wealth of discussion on the
topic. The frequency with which most code hits the problem is small, but
the effect is big enough to wreck tight real-time operation.
Regards,
Steve |
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Roman Rumian
Guest
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| Posted:
Tue Dec 20, 2005 12:17 am Post subject:
Re: Audio processing -- fixed or floating point? |
|
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Hi Vladimir,
Vladimir Vassilevsky napisa³(a):
(...)
| Quote: | Technically speaking, the single precision IEEE-754 float accuracy may
not be sufficient for the certain aspects of audio processing. The
32-bit int may produce better results.
|
thats interesting for me. I have problems with IIR filter stability (6th
order Butterworth highpass at 20 Hz and 48ksps sampling) on Motorola
DSP56300 (24-bit fixed point). Plan to change platform to SHARC Melody,
but it seems that should still use fixed point representation ? !
Best regards
Roman Rumian |
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Vladimir Vassilevsky
Guest
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| Posted:
Tue Dec 20, 2005 12:59 am Post subject:
Re: Audio processing -- fixed or floating point? |
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Roman Rumian wrote:
| Quote: | Technically speaking, the single precision IEEE-754 float accuracy may
not be sufficient for the certain aspects of audio processing. The
32-bit int may produce better results.
thats interesting for me. I have problems with IIR filter stability (6th
order Butterworth highpass at 20 Hz and 48ksps sampling) on Motorola
DSP56300 (24-bit fixed point).
|
It is quite obvious that is not possible to straightforward implement
any good IIR HPF with Fc = 20Hz Fsa = 48kHz using 24-bit precision. You
have to apply noise shaping or better use double precision ariphmetics.
| Quote: | Plan to change platform to SHARC Melody,
but it seems that should still use fixed point representation ? !
|
The IEEE-754 4-byte float has only 25 bits of precision, so it is not
going to help any. Indeed if you do MACs in float, it is going to be
much worse then using int 56-bit accumulation.
However SHARC native is 40-bit floating point which gives you extra 8
bits, which could be barely sufficient for what you are trying to do.
SHARC supports for 32 bit integer MACs with 80 bit accumulation. This is
the way to go.
Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com |
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Al Clark
Guest
|
| Posted:
Tue Dec 20, 2005 1:16 am Post subject:
Re: Audio processing -- fixed or floating point? |
|
|
"robert bristow-johnson" <rbj@audioimagination.com> wrote in
news:1135023945.749309.153870@o13g2000cwo.googlegroups.com:
| Quote: |
Vladimir Vassilevsky wrote:
The IEEE-754 4-byte float has only 25 bits of precision, so it is not
going to help any.
it will an improvement over fixed point for low-level signals.
Indeed if you do MACs in float, it is going to be
much worse then using int 56-bit accumulation.
depends on how loud the signal is.
However SHARC native is 40-bit floating point which gives you extra 8
bits, which could be barely sufficient for what you are trying to do.
with those 8 extra bits, you should be able to, on average, accumulate
256 terms in the 40 bit accumulator and have, on average, before a
noticeable bit error in the 32-bit float that you're rounding to,
SHARC supports for 32 bit integer MACs with 80 bit accumulation. This
is
the way to go.
Al Clark might agree with you.
|
I do.
Al
--
Al Clark
Danville Signal Processing, Inc.
--------------------------------------------------------------------
Purveyors of Fine DSP Hardware and other Cool Stuff
Available at http://www.danvillesignal.com |
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robert bristow-johnson
Guest
|
| Posted:
Tue Dec 20, 2005 1:16 am Post subject:
Re: Audio processing -- fixed or floating point? |
|
|
Vladimir Vassilevsky wrote:
| Quote: |
The IEEE-754 4-byte float has only 25 bits of precision, so it is not
going to help any.
|
it will an improvement over fixed point for low-level signals.
| Quote: | Indeed if you do MACs in float, it is going to be
much worse then using int 56-bit accumulation.
|
depends on how loud the signal is.
| Quote: | However SHARC native is 40-bit floating point which gives you extra 8
bits, which could be barely sufficient for what you are trying to do.
|
with those 8 extra bits, you should be able to, on average, accumulate
256 terms in the 40 bit accumulator and have, on average, before a
noticeable bit error in the 32-bit float that you're rounding to,
| Quote: | SHARC supports for 32 bit integer MACs with 80 bit accumulation. This is
the way to go.
|
Al Clark might agree with you.
r b-j |
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Jon Harris
Guest
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| Posted:
Tue Dec 20, 2005 9:15 am Post subject:
Re: Audio processing -- fixed or floating point? |
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Are you breaking your 6th order filter into 3 second order sections (in
series/cascade)? If not, that is the first step. If you are already doing
that, as an alternative to throwing more bits at it, you could also try a
different filter topology. I find that the "4-multiply normalized
lattice-ladder" structure is much better in terms of stability.
--
Jon Harris
SPAM blocker in place:
Remove 99 (but leave 7) to reply
"Roman Rumian" <usun_torumian@agh.edu.pl> wrote in message
news:do6tf9$vfc$1@news.agh.edu.pl...
| Quote: | Hi Vladimir,
Vladimir Vassilevsky napisa³(a):
(...)
Technically speaking, the single precision IEEE-754 float accuracy may not be
sufficient for the certain aspects of audio processing. The 32-bit int may
produce better results.
thats interesting for me. I have problems with IIR filter stability (6th order
Butterworth highpass at 20 Hz and 48ksps sampling) on Motorola DSP56300
(24-bit fixed point). Plan to change platform to SHARC Melody, but it seems
that should still use fixed point representation ? !
Best regards
Roman Rumian |
|
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Tobias Erichsen
Guest
|
| Posted:
Tue Dec 20, 2005 5:16 pm Post subject:
Re: Audio processing -- fixed or floating point? |
|
|
"john" <johns@xetron.com> schrieb im Newsbeitrag
news:1134909175.895541.297360@g49g2000cwa.googlegroups.com...
| Quote: | Not being an audio person, I was wondering what some of the experts
here would say about that assertion. Is it really always better to use
floating point for audio processing such as SRC, mixing, eq, etc?
|
I'd say that all current DAW (digitial-audio-workstation) applications for
PCs or Macs
do in fact already use floats instead of integer.
One reason is the fact, that float-performance on all PC CPUs in the last
couple of
years was way ahead of integer. And obviously you will get quite a bit more
precision this way.
One of the posters mentioned "denormals" as a performance-problem. This was
in fact a common thing which became aware with the introduction of P4 CPUs
(if I remember correctly). All of the DAW-software-manufacturers and
vendors of software-effects & instruments have since then done quite a lot
of
work that those denormals won't cause performance-issues any more...
Some effect-manufacturers have even started to use 64bit floats for internal
effect-processsing which is supposed to achieve even higher audio-quality.
But there a lot of people in that field seem quite esotheric about how
something
has to sound ;-) lamenting on how everything was better in those old analog
times ;-)
Tobias |
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Jerry Avins
Guest
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| Posted:
Tue Dec 20, 2005 5:16 pm Post subject:
Re: Audio processing -- fixed or floating point? |
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Tobias Erichsen wrote:
...
| Quote: | I'd say that all current DAW (digitial-audio-workstation) applications for
PCs or Macs do in fact already use floats instead of integer.
One reason is the fact, that float-performance on all PC CPUs in the last
couple of years was way ahead of integer.
And obviously you will get quite a bit more precision this way.
|
What makes that obvious?
...
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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Tobias Erichsen
Guest
|
| Posted:
Tue Dec 20, 2005 5:16 pm Post subject:
Re: Audio processing -- fixed or floating point? |
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|
"Jerry Avins" <jya@ieee.org> schrieb im Newsbeitrag
news:b62dnc6euJjbsDXeRVn-vA@rcn.net...
| Quote: | What makes that obvious?
|
Well - the original poster was referring to 16bit integer applications and
32bit floating-point
apps... so how would that not be obvious?
Tobias |
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Jerry Avins
Guest
|
| Posted:
Tue Dec 20, 2005 5:16 pm Post subject:
Re: Audio processing -- fixed or floating point? |
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Tobias Erichsen wrote:
| Quote: | "Jerry Avins" <jya@ieee.org> schrieb im Newsbeitrag
news:b62dnc6euJjbsDXeRVn-vA@rcn.net...
What makes that obvious?
Well - the original poster was referring to 16bit integer applications and
32bit floating-point
apps... so how would that not be obvious?
|
The difference is the bit count, not whether the binary point floats.
That wasn't obvious to me from what you wrote.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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