Digital Room Correction Project

[boconnor]

Originally Posted by Wingfeather

Hi all.....Whew, that's a long post!

And a very informative and helpful one! Thanks for posting it, its clarified a number of things for me.

So, I guess I’m likely to need to fix-up the latency difference between the video and audio. Any recommendations on a value-for-money unit that can delay video?

Originally Posted by lazycatken

I tried to apply as strong as possible correction to the filter of my system, until pre-echo is perceivable. A quick way to do this is to find how sensitive is your system vs pre-echo. Then make simulations, find the most possible strong correction. And verify it.

That seems sensible.

Check DRC document, you can find normalization factor... Balance, means, equal loudness of left & right channel. It's judged by ears. You know human ears are not frequency linear... One of the correction of DRC is magnitude frequency response. That is, DRC does the inverse of your system response.

Your post led me to re-read the DRC documentation. Yes, it does turn out that manual rebalancing is required once the filters have been created (as a final step). So the DRC program will make the frequency response of each speaker almost identical, but the relative signal magnitude between speakers may be different.

[Carl_Huff]

FYI,

Here is a pointer to two new commercial DSP room correction products. 'Dirac Live' on the AP20 & Dirac one

[Wingfeather]

Originally posted by boconnor
Any recommendations on a value-for-money unit that can delay video?

I've never seen any, though, it would require a lot of memory so I think embedded systems would be expensive. If you're using a PC for your source though, you're in luck - I believe that most software video players allow you to adjustably delay the video specifically for lip-sync purposes.

[boconnor]

Originally Posted by Wingfeather

I've never seen any, though, it would require a lot of memory so I think embedded systems would be expensive. If you're using a PC for your source though, you're in luck - I believe that most software video players allow you to adjustably delay the video specifically for lip-sync purposes.

I'm just using a DVD player for playback so this might get tricky about fixing the latency issue.

[dviswa]

Wingfeather,
Thanks a lot for a very informative post.

boconnor,
Good luck with your setup, I hope the latency does not get to be too much.

[boconnor]

I’ve been thinking about the options to deal with the audio latency issues highlighted in previous posts on this thread. I see three options at present:

1. Only FIR filter the squeezebox outputs, leaving the home cinema audio channels to go through my current 1/3 octave eq setup, which has no latency. This has the advantage of retaining the investment in existing DVD players etc and dealing with the latency by effectively bypassing it. The disadvantage is the need to have remote controlled switches to route the output signals from either the BruteFIR box in the case of music, or from the home cinema decoder. Also, it means I would have inferior sounding audio when playing music videos - although I’m not sure if that’s so much of an issue given the quality of audio mixing on many DVDs.
2. FIR filter all audio channels, and purchase a video delay unit to match the audio latency through the filters. Major disadvantage of this is the time to source a suitable video delay unit and the subsequent cost to purchase. New units on eBay by Rane, which offer variable delays and delay of PAL signals, look to be about US $800 – not cheap. There doesn’t seem to be much of a second hand market for variable delay units. I have seen some fairly cheap second hand units that do fixed delay (eg 2 frames of video) but I would have to know exactly how much delay the BruteFIR program is producing and then do matching compensation to get the audio and video lined up – doesn’t sound easy.
3. Change the hardware architecture – replace the existing PVR and DVD players with a properly tailored PC with high def tuners and video cards, so that video signals are captured within the PC and then can be delayed. Major advantage of this is that I can then FIR all audio outputs (music and home cinema). Disadvantage is cost and time as this would require replacement of the existing video systems.

Decisions, decisions.

[dviswa]

Originally Posted by boconnor

[*]Change the hardware architecture – replace the existing PVR and DVD players with a properly tailored PC with high def tuners and video cards, so that video signals are captured within the PC and then can be delayed. Major advantage of this is that I can then FIR all audio outputs (music and home cinema). Disadvantage is cost and time as this would require replacement of the existing video systems.[/LIST]

boconnor,
Capturing video signal on a pc means that you will be taking it from analog to digital and then back again? That might mean exchanging one devil with another, Audio quality will be exchanged for video quality. It would be good if you could take Video in digital domain into a PC, delay it and send it back out in digital domain.

Originally Posted by boconnor

Decisions, decisions.

Indeed.

[boconnor]

I have thought about the best way forward. Because I like to fully utilise existing equipment, and get value for money for my expenditure, I have decided to keep my existing video gear, and not to buy a video delay unit. So the current plan is:

FIR filter all audio (both music and from the home cinema). Check sync between video image and audio. If there is a sync problem, then change the BruteFIR configuration file to not filter the home cinema audio except for the sub-woofer channel, which will always be filtered. On balance this seems reasonable given the quality of audio on DVDs.

Any delay in the sub woofer channel is fine because I need to delay the sub woofer channel in any case, since it is too close to the listening position relative to the front speakers. I also need to deal with the sub 100Hz room modes.

The trick here is to get the overall delay right, which combines the inherent delay of the filter with the delay required because of physical distance between speakers.

So, if anyone has an easy method to determine the FIR filter delay through BruteFIR (I need a method that provides the actual millisecond number), please jump in.

[Wingfeather]

Hmmm, there's the `manual' method:
- I believe that both BruteFIR's block latency and the sound card's latency should be choosable/reported to you.
- If you have a filter file and some means to graph it (Excel, et all might work? MATLAB definitely does), then you can eyeball it for the first large peak.

Just add all those up and I think you should get your answer. If it's in samples then of course just divide by sample rate to get a value in seconds.

If you don't have the means to look at the filter file yourself then I'll be happy to do it - just send it over to me.

[boconnor]

Originally Posted by Wingfeather

Hmmm, there's the `manual' method: - I believe that both BruteFIR's block latency and the sound card's latency should be choosable/reported to you. -

Eh… a good reminder of the importance of reading the manual.

So I looked more closely at the BruteFIR documentation, which of course turns out to be a more useful thing to do than the cursory glance I gave some sections of the documentation when I first read them!

It looks like changing a parameter called the ‘partition size’ can vary the latency (delay), although the latency does also depend on hardware performance of RAM and other factors. It also turns out that BruteFIR can produce benchmark statistics. A configuration file with a specified filter is provided where you can check actual latency results against a table in the documentation.

Ominously, there is this in the documentation: “If you want to run BruteFIR to achieve high throughput, you should expect to have a delay of at least 100 ms …"

Like most things I guess I will only really know by applying my own filters with my own hardware. Although for my soundcard there appears to be a restriction on the partition size that can be specified (RME Digi9632 - max 8 partitions? – need to check the manual on that ). So before actually doing some measurements I suspect the latency will be above the threshold that has been mentioned earlier that causes problems in syncing audio with the video images.

Still, I’ve got some things to get right before we get to the point of doing actual measurements with filters. I’m having all sorts of problems getting Linux installed on the mini PC box I purchased. But that, as they say, is another story ...