Question: 32-bit DSP to 16-bit output: truncated?

shlopez · Jul 5, 2005, 04:56 PM

I have an EMU APS.

As I understand it, the DSP accepts 16-bit inputs and then performs its operations (routing, effects, mixing) at 32 bits internally.

But the outputs, like the inputs, are 16-bits, right? My question is, how does the DSP convert its 32-bit internal data back to 16 bits for output? Does it simply truncate the data? If so, is there a DSP plugin for dithering which could be inserted just before the output stage?

I've searched the forums and haven't found an answer to this question. Apologies in advance if I've missed it. Any info appreciated.

bennetng · Jul 5, 2005, 09:00 PM

If you are using coaxial output, the signal+effect is higher than 16-bit, this picture is a proof, the signal is recorded by another 24-bit audio interface from the coaxial output of sblive.

http://bennetng.kc-studio.com/sc-d70.../live_midi.png

However, if you are recording internally in 16-bit, then you need dither.

Download the dither plugin written by JoshuaChang
http://kx.cc21.com.cn/fx/dither.rar

or do it in a stupid way like this

http://bennetng.kc-studio.com/kXTut/Dither.png

Internal recording results... without dither
http://bennetng.kc-studio.com/kXTut/kX-No-Dither.htm

with dither...
http://bennetng.kc-studio.com/kXTut/kX-Dither.htm

shlopez · Jul 5, 2005, 09:20 PM

Thanks for your reply!

1. I don't understand how the coax output could be "higher than 16-bit." Are you saying that the APS is actually sending longer words? As I understand from reading the documentation on this site and elsewhere, the 10k01 chip has only 16-bit signal paths in and out, and I've not seen any reference anywhere to the idea that the coax outputs on the APS are anything but 16-bit.

2. Thanks for the info about the dither plugin but the link you provided for it is broken. I've also tried the link joshuachang supplied in his original post and it too is broken. Anyone else know where this plugin can be found?

bennetng · Jul 5, 2005, 10:09 PM

1. It's true! (at least for my live) see the screenshot, it says 24-bit...
http://bennetng.kc-studio.com/spdif.png

you can actually examine the recorded waveform and hear it! 16-bit recordings can't have signals below -96dB (you need a 24-bit compatible wave editor to open this file)
In fact the quality is not as high as 24-bit, but close to 20-bit already

http://bennetng.kc-studio.com/live_midi.rar

Let me explain a little bit more...sblive can only playback wave files in 16-bit resolution. If no effect is added, the recorded waveform will only have 16-bit resoulution even if 24-bit wave file is being played. However, if effects are being added, then the "effects" iteslf can have "higher than 16-bit" resoulution. Not only kX, even Creative drivers can output "higher that 16-bit" effects.

2. download from my homepage...
http://bennetng.kc-studio.com/dither.rar
ps: JoshuaChang says that in order to activate the plugin, users may need to copy the dither.da file to windows\system32 folder and double click it.

shlopez · Jul 6, 2005, 03:03 PM

Wow, interesting. My inspection of the file you provided does show that there are individual nonzero samples quieter than -96 dB. In fact, I found one at -117 db. That's essentially 20-bit resolution as you say.

OK, here's another question then. Suppose I have 16-bit tracks in Cubase (5.0 r1). Cubase mixes them in software at 32-bit resolution and sends the mix to FXBus 0/1, where it enters the DSP (in my setup) via an SRC plugin. Is the DSP actually getting 32 bits, or can it accept only 16?

If the latter, then it would seem that I should turn dither on in the main mixer window of Cubase, so that Cubase itself will dither from 32 to 16 bits instead of truncating. Then this would insure that the DSP got the best possible 16-bit data on input.

Then if I understand correctly, if I now use the DSP to mix audio and midi streams and add effects, the resulting 32-bit mix will be (a) truncated to 16 at analog outs (so using dither plugin is recommended) but (b) the coax digital outs will still contain 20 bits of audio information.

If I understand correctly however, all this means that when I play back multiple tracks in Cubase and listen via the analog outs, I'm hearing 16-bit output that has been TWICE converted to 32 bits, processed, and truncated (or dithered) back to 16 bits (once in Cubase, then once more in the DSP) before reaching the DAC.

Do I have this right?

Thanks for your help!

bennetng · Jul 6, 2005, 03:39 PM

The DSP only accepts 16-bit, so it is a good idea to dither the signal to 16-bit before entering kX DSP.

For analog output, it depends on the DAC chip of your APS. If it supports input higher than 16-bit, then dither is not necessary. I can't tell if dither can improve analog quality or not because the cheap Sigmatel codec of my SBLive is already noisy enough so I can't measure the differences between dithering and truncation.

shlopez · Jul 6, 2005, 05:25 PM

I believe the ADC and DAC on the APS are 20-bit. The ADC converts at 20 bits and dithers to 16; the DAC converts at 20 bits to analog. Perhaps this is why the DSP outputs at 20 bits and not to 16! That would make the most sense for feeding a 20-bit DAC...

The following link is to a Mix Magazine review of the APS I found which mentions that the S/PDIF outs are set to 20 bits.

http://mixonline.com/mag/audio_emu_audio_production/

Thanks for the information you provided -- it's good to understand better what's happening to the audio as it goes through the signal path.

Jul 5, 2005, 04:56 PM	#1
shlopez DriverHeaven Newbie Join Date: Jun 2005 Posts: 5 Rep Power: 0	Question: 32-bit DSP to 16-bit output: truncated? I have an EMU APS. As I understand it, the DSP accepts 16-bit inputs and then performs its operations (routing, effects, mixing) at 32 bits internally. But the outputs, like the inputs, are 16-bits, right? My question is, how does the DSP convert its 32-bit internal data back to 16 bits for output? Does it simply truncate the data? If so, is there a DSP plugin for dithering which could be inserted just before the output stage? I've searched the forums and haven't found an answer to this question. Apologies in advance if I've missed it. Any info appreciated.

Jul 5, 2005, 10:09 PM	#4
bennetng DriverHeaven Junior Member Join Date: Jan 2004 Location: Hong Kong Posts: 61 Rep Power: 0	1. It's true! (at least for my live) see the screenshot, it says 24-bit... http://bennetng.kc-studio.com/spdif.png you can actually examine the recorded waveform and hear it! 16-bit recordings can't have signals below -96dB (you need a 24-bit compatible wave editor to open this file) In fact the quality is not as high as 24-bit, but close to 20-bit already http://bennetng.kc-studio.com/live_midi.rar Let me explain a little bit more...sblive can only playback wave files in 16-bit resolution. If no effect is added, the recorded waveform will only have 16-bit resoulution even if 24-bit wave file is being played. However, if effects are being added, then the "effects" iteslf can have "higher than 16-bit" resoulution. Not only kX, even Creative drivers can output "higher that 16-bit" effects. 2. download from my homepage... http://bennetng.kc-studio.com/dither.rar ps: JoshuaChang says that in order to activate the plugin, users may need to copy the dither.da file to windows\system32 folder and double click it. Last edited by bennetng; Jul 5, 2005 at 11:07 PM.

Jul 5, 2005, 09:00 PM	#2
bennetng DriverHeaven Junior Member Join Date: Jan 2004 Location: Hong Kong Posts: 61 Rep Power: 0	If you are using coaxial output, the signal+effect is higher than 16-bit, this picture is a proof, the signal is recorded by another 24-bit audio interface from the coaxial output of sblive. http://bennetng.kc-studio.com/sc-d70.../live_midi.png However, if you are recording internally in 16-bit, then you need dither. Download the dither plugin written by JoshuaChang http://kx.cc21.com.cn/fx/dither.rar or do it in a stupid way like this http://bennetng.kc-studio.com/kXTut/Dither.png Internal recording results... without dither http://bennetng.kc-studio.com/kXTut/kX-No-Dither.htm with dither... http://bennetng.kc-studio.com/kXTut/kX-Dither.htm

Jul 5, 2005, 09:20 PM	Thread Starter #3
shlopez DriverHeaven Newbie Join Date: Jun 2005 Posts: 5 Rep Power: 0	Thanks for your reply! 1. I don't understand how the coax output could be "higher than 16-bit." Are you saying that the APS is actually sending longer words? As I understand from reading the documentation on this site and elsewhere, the 10k01 chip has only 16-bit signal paths in and out, and I've not seen any reference anywhere to the idea that the coax outputs on the APS are anything but 16-bit. 2. Thanks for the info about the dither plugin but the link you provided for it is broken. I've also tried the link joshuachang supplied in his original post and it too is broken. Anyone else know where this plugin can be found?

Jul 6, 2005, 03:03 PM	Thread Starter #5
shlopez DriverHeaven Newbie Join Date: Jun 2005 Posts: 5 Rep Power: 0	Wow, interesting. My inspection of the file you provided does show that there are individual nonzero samples quieter than -96 dB. In fact, I found one at -117 db. That's essentially 20-bit resolution as you say. OK, here's another question then. Suppose I have 16-bit tracks in Cubase (5.0 r1). Cubase mixes them in software at 32-bit resolution and sends the mix to FXBus 0/1, where it enters the DSP (in my setup) via an SRC plugin. Is the DSP actually getting 32 bits, or can it accept only 16? If the latter, then it would seem that I should turn dither on in the main mixer window of Cubase, so that Cubase itself will dither from 32 to 16 bits instead of truncating. Then this would insure that the DSP got the best possible 16-bit data on input. Then if I understand correctly, if I now use the DSP to mix audio and midi streams and add effects, the resulting 32-bit mix will be (a) truncated to 16 at analog outs (so using dither plugin is recommended) but (b) the coax digital outs will still contain 20 bits of audio information. If I understand correctly however, all this means that when I play back multiple tracks in Cubase and listen via the analog outs, I'm hearing 16-bit output that has been TWICE converted to 32 bits, processed, and truncated (or dithered) back to 16 bits (once in Cubase, then once more in the DSP) before reaching the DAC. Do I have this right? Thanks for your help!

Jul 6, 2005, 03:39 PM	#6
bennetng DriverHeaven Junior Member Join Date: Jan 2004 Location: Hong Kong Posts: 61 Rep Power: 0	The DSP only accepts 16-bit, so it is a good idea to dither the signal to 16-bit before entering kX DSP. For analog output, it depends on the DAC chip of your APS. If it supports input higher than 16-bit, then dither is not necessary. I can't tell if dither can improve analog quality or not because the cheap Sigmatel codec of my SBLive is already noisy enough so I can't measure the differences between dithering and truncation.

Jul 6, 2005, 05:25 PM	Thread Starter #7
shlopez DriverHeaven Newbie Join Date: Jun 2005 Posts: 5 Rep Power: 0	I believe the ADC and DAC on the APS are 20-bit. The ADC converts at 20 bits and dithers to 16; the DAC converts at 20 bits to analog. Perhaps this is why the DSP outputs at 20 bits and not to 16! That would make the most sense for feeding a 20-bit DAC... The following link is to a Mix Magazine review of the APS I found which mentions that the S/PDIF outs are set to 20 bits. http://mixonline.com/mag/audio_emu_audio_production/ Thanks for the information you provided -- it's good to understand better what's happening to the audio as it goes through the signal path.