My Simple Surround System

I started fooling around with Surround Sound in 2005, and that's about all it was, fooling around.  I had purchased 5 speakers from my brother-in-law, and I experimented with placing them on the back counter of my kitchen, where they quickly got in the way and had to be removed.

The current system really got going around 2018 or so, but it's still based on my 2005 Yamana HTR-5790 receiver, which has always worked very well for stereo and everything.  It's got very good amplifiers (for 7 channels!) and good performance everywhere.  It can decode common surround signals from coax and optical connections.  What it doesn't have is HDMI inputs and the ability to receive surround sound via HDMI.

That hasn't been an impediment, since I can either take the coax low res digital straight from my Oppo BDP-95 for movies, or the seven channel analog output from the Oppo for high resolution audio multichannel discs.  Normally I just use the analog for everything, so all my digital decoding is done by the Oppo.  The only way this is a "problem" is that I'm not getting high resolution (from high resolution audio discs, not movies) from the few high resolution discs I have in pure digital form for the fancy processing in the receiver.  But that doesn't matter much, since the Yamaha receiver doesn't do fancy processing on the seven channel analog inputs.  Instead, I do all the basic digital processing required in the Oppo, which has menus for setting the delays and levels for each channel.  So the only way that this is a "problem" is that I'm not getting all the fancy digital processing applications like Audessy, that may correct for EQ and phase.  And I wouldn't even want Audessy much, except the later versions which allow you to do the fine tuning on your phone.  And that set of Home Theater Processors are still very expensive even used, because they have that feature.

Anyway, I'm getting the seven channels and routing them to amplifiers in the Yamaha and thence to speakers.  With levels and delays set in the Oppo.  That's pretty much all there is to it.

Except, what about when there is 5.1 channel content?  I could play it back just as that, which is theoretically fine.   But because my side speakers are not optimally placed, I often find it better to play 5.1 channels on my 7.1 speakers by duplicating the side content in the back.  When I do that, I lower the level by 2dB so it still balances OK.  I use a rotary switchbox (made by dB systems no less) to switch from pure discrete 7.1 to 5.1 expanded to 7.1.  And then I have a preamp for the sides having a level control with two marked positions.  I use the higher position for discrete 7.1 and the lower one for fake 7.1.

My project to add optimally placed side speakers has gone nowhere in 5 years since I bought the required wall hangable small speakers, currently still in my bedroom to remind me to install them.

(I have tried many other solutions for the 5.1 to 7.1 conversion.  For some time I used a well known box from the  1980's.  After using it a year I found it distorted the sound intolerably.  Then I tried using various delays and EQ's.  Finally I decided that simply duplicating the sides in the backs, and lowering the level by 2dB, worked better than anything else.  I also tried a historic Integra processor, but found it did nothing useful, and was basically a pain in the neck because it only produced distortion if input or output levels exceeded 1 volt.  The Yamaha is good at least to 3 volts on inputs and outputs.)

I could solve this 5.1 to 7.1 conversion problem problem "better" with a fancy Home Theater Processor that might cost $5000 (do they still make those???) or more, but it's not been worth it to me when I could have afforded it (and now it's simply unaffordable).

Anyway, my simple idea is implemented with the units on top of my kitchen rack.  The bottom DEQ box is for the rear speakers and it only does level adjustment for the backs, plus level and spectrum displays (so you can see at a glance if the backs are doing anything).  The level adjustment feature that this box actually does could just as well be done by a preamp (like the upper two boxes) but I happened to have the DEQ and not another preamp.  (It inherently adds about 10mS of additional latency, which I have compensated for in the Oppo adjustments.  When used for the fake 7.1 the additional delay is a bonus that makes it sound a bit better--like an even larger room--but isn't that important either way)

The upper DEQ box is for the subs and currently does nothing more than level and spectrum display.  (I thought it was also doing some eq but it appears not.  At some point in the last couple years I bypassed it.  I'm not sure that was out of design or necessity--such as it might have been adding hum).  Currently the upper DEQ box is non-functional, it appears to have the usual power supply issue.  But since I'm not doing any processing there anyway, it doesn't affect the sound.  But it was very useful to have the spectrum display on the bass because I could measure ground loops exactly and work to eliminate them.  The spectrum display on the subwoofer signal enabled me to solve and fix about a dozen ground loops over the years.  The complicated kitchen electronics (my central computer, video,  and audio for the whole house, plus TV and radio) are prone to those.  Now the box that did the helpful bass spectrum displays needs fixing.

The upper two preamp boxes are for the sub and sides.  It allows convenient setting of the levels, which I need to do for the sides when changing from discrete 7.1 to fake 7.1.  Also historically I used to mess with the sub level a lot depending on recording.  But I've had it dialed in pretty well for everything recently and hardly mess with it at all anymore.  Still I like have "controls" that I can just reach up and control as opposed to complex apps which may not do anything until you take another measurement.

The stereo frequency response (including subs) is quite flat, though I think some of my low frequency optimizations that used to live in the subwoofer DEQ would have made it flatter there.  Still it sounds pretty good.

Rethinking the Oppo BDP-205 Filter Choice

My last post got 'updated' with a long discussion and testing of the Oppo BDP-205 filter choices.  I had been using the Linear Fast filter, thinking that was the best of the best.  But it seemed to induce about 5 clipping events beyond +4dB headroom I allowed for inter sample overs (ISOs), which shouldn't be happening at all (or at least Benchmark seemed to say it was only necessary to allow 3.1dB headroom for inter sample overs).  This is not necessarily the flaw of the Oppo or any of it's digital filters, but a deviance from my previous expectation and a seeming objective variance between the different filter choices.  (See update below.  I have subsequently determined that my counts of clipping events was wrong.  It looks like all the fast filters generated the same number of clipping events.  They just looked different.)

The other choice that looked best to me at that point (on technical considerations and examining Archimago's measurements) was the Brickwall.  I found that only induced 1 clipping event above +4dB, thereby seeming objectively better, plus having the best numerical specifications on noise, distortion, and loss at 20kHz.

I'm NOT going to choose any of the slow filters even if they eliminated such clipping events (in fact, I'd expect they might) because of their leakage effects.

But there's one other filter that might be the best of all, and it's actually Oppo's choice for the default, so it's apparently what they thought to be the best.  It's the Minimum Phase Fast filter.

This filter has the curious effect, like all other minimum phase filters, of moving the ultrasonic ringing past any transient, rather than being on both sides (acausal).  Intuitively this seems better to most, including me.  I was brushing it off last time as "unnatural" but in fact it is the natural thing you could get with real circuits rather than digital simulations, if you could make those circuits well enough (which in practice isn't possible).

We'll I'm long past thinking about ultrasonic ringing in the first place.  I'm more interested in low noise, extended response, and those being equal I'd consider the phase response.

The minimum phase fast HAS higher noise and distortion than the brickwall...but also it appears to have more ultimate bandwidth (a big plus) at least according to Archimago's spectrum graphs (which didn't look right, because the brickwall had the lowest loss at 20k, but on the graph it was cutting out well before 20k steeply).  So given the possibility that min phase fast has the widest bandwidth, and nearly as low distortion, that could make it the best.

Anyway it occurred to me it might not have these these above 4dB inter sample overs in the first place, and if so it would be an obvious choice.

But the test shown below shows it has the same single clipping event over +4dB as the brickwall.  So it's "equal" in that respect, and better than the linear phase which had 5 such events.

But...looking at that actual clipping event...it makes far more sense than with any of the other filter, to my analytical eye.  In fact it makes so much sense, I'm inclined not to "repair" it at all, as there's no repair that would preserve the underlying high frequency transient it is apparently trying to show (which previously I insisted had to be electronic...and it might be...but that hardly matters here) without smearing it.

It looks to me best just left very slightly clipped, for it's clear the clipping is right at the upper bound of where it's going to be anyway.  There's likely so little difference made in the single clipping event it's not worth lowering the recording level -0.5dB to avoid it (though a test might be warranted).  The clipping looks to be hardly making a difference...the peak would only reach a microscopic amount higher anyway, judging from the trailing ringing which is now concentrated on the "after" side making it easier to understand.  The other filters give very messy looking results that defy repair altogether, compared to how this looks.

Min Fast +4db clip (look for the ringing)

I'm now leaning towards leaving the Oppo at the factory default Min Fast setting.

Update:

My counts of clipping events may have been wrong.  I was just checking visually.  This can be misleading and depend on how much time is being displayed and also when exactly it starts.  I discovered this by cutting the first section of the Min Fast recording and discovering that there were no clipping events at all, instead of just one.  The one had disappeared because of the differing start time.  Then when I magnified and scrolled, I saw several events.  I probably made that exact same mistake with Brickwall.  Lets assume for now they all have the same number of clipping events.  (That also explains a recollection of another file I didn't report.  It was also Brickwall but had several clipping events, just like linear fast.)

So this number of clipping events was an entirely bogus analysis.  What still looks good is the concentrated and natural way the Min Fast makes each inter sample over clipping event look.  They just look right, whereas all the others just looked awful.

Right now I have no evidence that the factory default filter isn't the best, and one subjective guess that it looks best.  (I doubt I could hear the difference...especially in a double blind test.)

Update May 17

I listened to Min Fast filter last night and thought it sounded great.  Pure, harmonic, and no digital artifacted sound.

But now it appears that my bit about the look of the ISO clipping events (once again, the fact that there is clipping is not Oppo's fault, it's mine) and it's damned hard to tell even which one looks best.  The Linear Phase Fast filter, which I now see is Archimago's preference, does give very short ringing, shorter even than Brickwall, though it has that annoying pre-ringing also.  Archimago simply argued on merits, that linear fast is like previous Oppos and most players, and admitted that he's never heard a difference among filters nor was a difference found in earlier testing of similar filters.

The Audioholics tester (Gene Dellasala) also tried very hard and could not hear a difference among the filters.  His advice was leave it at the factory setting (Min Fast) and worry about more important things.

The difference in noise among the Brickwall, Min Fast, and Lin Fast filters is negligible.  The Brickwall noise level is slightly better with -119.0 instead of -118.8.  The THD numbers are identical at a barely measurable 0.0008%.  That indicates none of these three filters have significant leakage.  The Brickwall apparently cuts off a microscopic amount faster somewhere above 20kHz resulting in the tiniest bit of extra noise reduction but the benefit is so small, one might as well use the more "natural" looking Min Fast, and maybe that slight added bandwidth is a good thing (especially for someone like me, using supertweeters).  The Min Fast may have the widest bandwidth as it does have the lowest loss at 20kHz (-0.19 for the Min Fast vs -0.21 for the Linear Fast vs -0.20 for the Brickwall), and since that's the one and only superior spec for that filter, perhaps it's why Oppo chose it as the default.

Actually none of this matters to me  anyway because I only use the Oppo to play high resolution discs, with sampling above 44.1kHz, or SACD's.  Standard discs I simply rip to my computer and send the CD quality bits into my system and the DAC in the Oppo doesn't matter at all.  With high sampling rates on high resolution discs and SACD's, any of these filters is way more than good enough.  I could even go with the slow filters and not suffer significant alias leakage.

I think I'm going to follow Gene Dellasala's advice, which I was trending to anyway.

I also don't have any MQA discs (do they exist?) and when I stream the Oppo DAC isn't involved (and I no longer use a streaming service that supports MQA either).  Some apparently like the Apodizing filter with MQA.  But for other uses, the Apodizing filter is unappetizing to me in general because it adds a tad of high frequency ripple (almost certainly inaudible, but why have it anyway).