Digital x/o with A/V Sources

zydeco

Well-Known Member
Oct 16, 2010
59
0
311
WA, Australia
I’m after advice on how to best incorporate a digital cross-over into a system that has a mixture of audio and video sources. The system comprises a range of A/V sources (CAPS | Apple TV | TiVo) with video sent to a TV and audio sent to a traditional DAC | Pre-Amp | Amp | Speaker chain. All this has evolved into a 2.4 set-up with subwoofers used to smooth room response with these fed with line-level signal from the pre-amp and on-board cross-over used. Over the past few months I’ve added room EQ for audio from the computer (CAPS / W8 / JRiver) source with FIR filters generated in Acourate but all other sources are untouched. The reason for the thread is that I’m using a digital cross-over to further optimise the integration of the subwoofers with the main speakers. I’m thinking that this means both time alignment (although I’m not sure if this is critical) as well as individual EQ of the subwoofers (in addition to the overall EQ via CAPS / JRiver). My two main questions are as follows:

1. What is meant by “time alignment of subwoofers”? Is it ensuring that the signal arrives at the main speakers at the same time as that from the main speakers? Or, alternatively, is it ensuring that all woofers fire at the same time? The latter makes more sense to me given that we’re taking about energising the room. And, if it's the case, then what's the benefit of a digital x/o assuming that all the subs are identical (and thus have same phase shift)?

2. And how is this best achieved in a multi-source A/V system? My preferred approach was to use a dedicated convolution computer with a multi-channel audio interface with either digital or analogue inputs as this would allow me to leverage strengths of Acourate. A quick trial with my current filters and AcourateConvolver found that the audio lag was much too long for this to work with A/V sources. Some options look to be:

a. Convolution computer with filters (FIR) with much fewer “taps”. I’m not that excited about this option as it is, effectively, degrading my main source (CAPS | JRiver) for the benefit of additional A/V sources.

b. Add a pro-audio digital x/o (e.g., Xicila XD-4080) after pre-amp. I’m not concerned about the A/D/D/A but am worried about audio / video sync and reliability in home set-up. Has anyone tried these units?

c. Add DEQX that replaces the DAC | Pre-Amp. This seems like a great solution but is at limit of budget.

It’d be great to get feedback on these two questions including thoughts on options for implementing a digital x/o.

Zydeco
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
Time alignment of subs = getting the signals from all of your different subs and mains to arrive at "roughly" the same time such that they add for smoothest bass with the least ringing. It's possible to align everything precisely yet it still sound better with some bass sound sources brought in phase lagging or leading (45 degrees is about as much as I'd recommend) because that way some of the dips are filled in.

Ideally you need ability to independently delay main speakers and each sub. If subs are equidistant from listening position then you can get away with only one delay for two subs. For example if you have four subs, two in front corners, two in back corners and are sat say 2/3rds of way back in room then you would need three channels of delay: one for mains, one for front subs, one for back subs.

In your situation I'd suggest a pro DSP is the easiest way to go. They add very little latency - say 2ms. The Xilica's are great, I use them all the time, with unbalanced or balanced inputs at all kinds of input levels. No noise, no hiss, just solid.

Obviously if you need a bunch of audio delay then you might get out of sync with video. However in my experience video processing delays are way way longer than what you would need to get everything time aligned. Anything from 70ms for a flat panel to 150ms for a projector. So mostly you'll need to delay the audio. You can measure AV sync using a test pattern such as the one on Disney WOW calibration disc.
 

zydeco

Well-Known Member
Oct 16, 2010
59
0
311
WA, Australia
Xicila FIR Filters

Time alignment of subs = getting the signals from all of your different subs and mains to arrive at "roughly" the same time such that they add for smoothest bass with the least ringing. It's possible to align everything precisely yet it still sound better with some bass sound sources brought in phase lagging or leading (45 degrees is about as much as I'd recommend) because that way some of the dips are filled in .

That’s interesting – and the bit about minor phase differences to smooth response makes sense. My rough estimate is that the difference in travel time will, at worst, create a 10ms lag (the furthest subwoofer is 3.5m further from the listening position than the main speakers). I seem to recall that an analogue or IRR low-pass filter will add another 10ms (from full range main speakers) which, if true, would see this subwoofer at 20ms behind the main speakers at 80Hz. Does this sound right? And is it correct to assume that it’s a large enough lag to bother correcting?

In your situation I'd suggest a pro DSP is the easiest way to go. They add very little latency - say 2ms. The Xilica's are great, I use them all the time, with unbalanced or balanced inputs at all kinds of input levels. No noise, no hiss, just solid.

That’s impressive. I’m thinking of using the FIR filters so as to ensure delays are independent of frequency and thus simplify time alignment correction. Is this the recommended approach? And what is the latency for these types of filters? The other concern is that having looked at the manual for the XD-4080 it looks as if the FIR filters have a small number of “taps” – are these filters sufficiently resolving?

Obviously if you need a bunch of audio delay then you might get out of sync with video. However in my experience video processing delays are way way longer than what you would need to get everything time aligned. Anything from 70ms for a flat panel to 150ms for a projector. So mostly you'll need to delay the audio. You can measure AV sync using a test pattern such as the one on Disney WOW calibration disc.

Thanks – I’d not realised that that was the case.

Cheers, Zydeco
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
1) The time delay added by an IIR filter depends on the slope, frequency and type of filter you are using, no hard and fast rules. Generally it's not something to worry about. You measure the mains and subs and from that work out the time delay required to align everything rather than trying to work it out on paper. And yes, 20ms would be a significant time delay in audio terms. You can normally see the effects of even a 1-2ms adjustment to the sub or mains on the smoothness of the frequency response of the combined sub+mains.
2) The XD is really designed to use IIR filters for the higher frequencies and IIR for the lower. You need a lot of DSP to do low frequency FIR filters, and they also add a lot of latency, hence more opportunity to screw up your video/audio sync.
 

zydeco

Well-Known Member
Oct 16, 2010
59
0
311
WA, Australia
And yes, 20ms would be a significant time delay in audio terms. You can normally see the effects of even a 1-2ms adjustment to the sub or mains on the smoothness of the frequency response of the combined sub+mains.
Is it correct that the sole measure of time alignment is the overall frequency response? Or, put another way, if one gets a good overall response via tinkering with phase of individual subwoofers then is there anything to be gained by individual time alignment?
 

Nyal Mellor

Industry Expert
Jul 14, 2010
590
4
330
SF Bay Area, CA, USA
Is it correct that the sole measure of time alignment is the overall frequency response? Or, put another way, if one gets a good overall response via tinkering with phase of individual subwoofers then is there anything to be gained by individual time alignment?

No, time alignment is important. Consider two speakers playing 100 Hz sine wave in phase yet 100ms apart. Frequency response will look perfect if phase is aligned yet speakers will not be time aligned. Time aligning them will not change frequency response but will make a substantial difference in perceived sound.
 

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