record for SVCD?

[pick-a-name]

Has anybody tried to set the tivo's recording parameters to exactly match what is needed to make a SVCD?

In other words, setup the mpeg encoder chip (CXD1922) registers to the exact bitrate required as well as the sound digitiser (not sure which chip this is) so that the extracted stream would not need to be re-encoded?
Such a stream may not play on the Tivo, but that would not matter if you could play a disk instead - right?
This seems to me to be the real holy grail for making SVCDs. It would for example be simple and fast to dump video from a camera into the tivo, have it encoded and suck it back out to burn to disk.

[sonnik]

The only mods I've known about for this is Tridge's hack to use with PAL vs. NTSC. I don't really think the codec itself could be modified.

The only probable way you could do different encoding is a lot of software hacks that do the encoding via software. And TiVo's processing power can't handle that.

[pick-a-name]

Check this out:
http://www.sel.sony.com/semi/PDF/SS_CXD1922.pdf
http://www.sel.sony.com/semi/PDF/CXD1922Q.pdf

According to this (limited) data sheet, the encoder can do any bit rate you want, MPEG1 or MPEG2 compatable. It is just a matter of setting the chip config registers to what you want.
I am quite certain that the audio encoder can also be set up to any desired spec, the the tivo playback system likewise can be configured to accept any valid setup. The only problem is figuring out how to set up the encoder. All the data sheet info for every chip inside the tivo is available online, except the CXD1922. Since tridge figured out how to do PAL, it seems like a small stretch to adjust other recording parameters.

Again, if this could be worked out, you could directly create MPEG source data exactly as needed to create VCD, SVCD or even DVD in real time.

[dopester]

I am certain with some tweaking similar to how you mod the tivo to record in pal you could mod the High Quality setting to change the bitrate to 2600 (Same as Medium Quality), but use 480x480 resolution (High Quality and SVCD Resolution)..

The main problem is the audio encoding chip on the tivo is fixed at 32khz. This is not SVCD quality, and upsampling will create distortion.. (It can be done with minimal distortion using a good interpolation, but I have yet to find one that doest create frequencies that are not encoded at 32kHz)..

So im not sure if that will stop your quest or not.. One thing that you might want to look into possibly is the DirecTivo.. The resolution is 480x480 and there is no green bar.. I have no idea how the audio is stored, but I am guessing its going to be 44.1k or 48k.. Either way you can get it back to 44.1k if need be quite easy.. The DirecTivo doesnt set the avg birtate since it just writes the mpegs from the birds to the harddrive so you might have to re-encode that video as well. but the quality is MUCH MUCH better than on the standalone since you arent re-encoding again..

[pick-a-name]

Originally posted by dopester

The main problem is the audio encoding chip on the tivo is fixed at 32khz. This is not SVCD quality, and upsampling will create distortion.. (It can be done with minimal distortion using a good interpolation, but I have yet to find one that doest create frequencies that are not encoded at 32kHz)..



That will be a problem. I have not looked into how audio is encoded. Which chip does the audio encoding anyway?
Are you sure there is no way to change it?

[Worf]

The ADSP 2105 DSP chip does the encoding. The algorithm itself is coded in somewhere and loaded into the DSP chip. Not sure if changing the sampling rate will require modifications to the encoding software itself, but it's highly likely it will.

[pick-a-name]

Originally posted by Worf
The ADSP 2105 DSP chip does the encoding. The algorithm itself is coded in somewhere and loaded into the DSP chip. Not sure if changing the sampling rate will require modifications to the encoding software itself, but it's highly likely it will.

I looked up those chips, and I do not see where the A to D takes place for audio.
The DSP appears to be doing some encoding, but the serial data coming into it I think is what needs to be modified. Not the DSP program.
Just from looking at chip data sheets, it looks to me like the SAA7114 video decoder has something to do with it. It generates an audio clock referenced to the video clock which is programmable via the I2C bus. maybe all that is needed is to bump up the audio clock rate to increase the sample rate?
If it is that easy, then maybe the DSP will simply react and do what we want?

Has anybody tried to make a schematic of this thing? That would come in handy now.

[TiVoTweakr]

Originally posted by dopester
I am certain with some tweaking similar to how you mod the tivo to record in pal you could mod the High Quality setting to change the bitrate to 2600 (Same as Medium Quality), but use 480x480 resolution (High Quality and SVCD Resolution)..



If one were doing so, it would be better to try to set it for XVCD or even SXVCD.

[pick-a-name]

I am convinced that we can record any video bit-rate or format that is legal under mpeg-1 or mpeg2. The cxd1922 data sheet says this is possible. What it does not say is how to do it. I can not get a real data sheet or user manual for this chip. Maybe by intercepting the I2C commands sent to the chip under the current recording choices we could make an educated guess about how to modify them.
That would not solve the audio sample rate problem but perhaps it would be easier to just capture the audio directly to the PC however you want and mix them together later. That kind of defeats the whole purpose of using the tivo, but the real time eater is the mpeg encoding anyway.
That would still be a better solution than any PCI card hardware encoder I have seen. (for under $4000 anyway)

[Delta]

Originally posted by pick-a-name
The cxd1922 data sheet says this is possible.

Just because a chip has a feature doesn't mean that the feature has been implemented in the circuit design. It is possible that the TiVo system board locks the audio sampling rate at 32K for a/v sync or other unthinkable reasons.

The whole point of a chip's data sheet is to show the designer whats possible. It's up to the designer to decide which functions they wish to use.

---

As a side note: Has anyone tried upsampling the audio to 48K instead of 44.1? My DVD player has trouble playing 32K VCD's but it will play 48K audio just fine. Also the distortion seems to be less when sampled to 48K instead of 44.1K. Just a thought...

[pick-a-name]

Originally posted by Delta


The whole point of a chip's data sheet is to show the designer whats possible. It's up to the designer to decide which functions they wish to use.



If you look at most of these chips, the functionality is controlled by how it is programmed via the I2C bus.
That is why people were able to re-purpose the tivo to do PAL. All the chips (except the tuner) were already capable of it and just needed to be re-programmed. That is why I think just changing the clock rate somewhere will do the trick for the audio sampler.
Of course there is a lot of wishfull thinking going on here for the audio part, but the video encoding parameters are almost certainly possible to adjust to whatever we want. Tivo does it now to get the different quality settings.

[pick-a-name]

I did a little poking around the audio DSP and found that it is clocked by the "micro-clock" chip. This chip has programmable rate select pins that are connected to some programmable I/O pins of the DSP - suggesting that the ability to change the sample rate is somewhere in the DSP program. (a already known fact right?) Checking around at the Analog Devices site, they talk about audio encoding software that comes with the development kit for this device. It is likely that tivo used this code as their base. Has anybody found the DSP program on disk that gets loaded into the DSP?

[laserfan]

This old thread deserves a bump I think. Not sure I've seen any of these guys around here lately, but the video resolution and bitrate questions have since been resolved (to a happy result) so I wonder where the magical/mystical audio sample rate setting occurs.

Wouldn't it be lovely if we SA owners could set our boxes to 44.1kHz (vcd, svcd) or 48kHz (dvd) sample rates?

This would likely require someone to deconstruct how the Tivo's DSP is programmed, an exercise way above my abilities...

[v8q]

Originally posted by laserfan
This old thread deserves a bump I think. Not sure I've seen any of these guys around here lately, but the video resolution and bitrate questions have since been resolved (to a happy result) so I wonder where the magical/mystical audio sample rate setting occurs.

Wouldn't it be lovely if we SA owners could set our boxes to 44.1kHz (vcd, svcd) or 48kHz (dvd) sample rates?

This would likely require someone to deconstruct how the Tivo's DSP is programmed, an exercise way above my abilities...

I have not touched my tivo innards since this thread was fresh - but I remain convinced (thru ignorance no doubt :) that the audio can be adjusted. My guess is that the DSP will simply respond the way we want if the upstream clocks and data are reset to 44K - as long as the audio encoding algorythm is the same for 32K and 44K hertz. As far as I could tell, this is all done via the I2C bus.
The only other possible show stopper is changing the clock that drives the DSP. It may be necessary to hardwire the clock driver chip rate inputs to obtain the desired clock rate - or else someone has to hack the DSP code since these lines are driven by some outputs from the DSP.
btw, this is the fact that makes me believe tivo left the audio encoding section programmable. Otherwise they would have simply hardwired the clock driver inputs themselves.

[lmurray]

i wanted this too, but... just gave in and bought an apex dvd player that can do the 32Khz audio. $60 at your local walmart.

If the audio conversion can be done, hopefully it can be incorporated into tivoweb.

-lloyd-