Nest Theater Part Twenty Four ~ Center Channel Experiments Take It Back To The Future 01Jun23

I am building a center channel and possibly surrounds for the Nest Theater. I first need to find a decent center channel source that will integrate well with the current stereo soundfield. I looked around the net for "cheap" gear which I could build or buy that would get me a configurable and flexible center channel with which to experiment. I chose to grab a used broadcast quality Dolby decoder named DP564 which is a fancy DAC & decoder supporting various speaker configurations for physically assuring quality listening is being sent on a broadcasters' feed. Current gen boxes run $4K but I found the prior generation with up to Dolby Digital and Pro Logic II (5.1) for much cheaper. This is now used to create a center channel (and possibly L/R surround channels) from digital 2 channel PCM or 5.1 channel DD sources. I found Chinese Dolby 5.1 decoders available for ~$50 but reviews were bad and I'm not sure they really work (would $50 even cover Dolby's license fee?)... my new Dolby decoder, will do Dolby. I also found plenty of Dolby-equipped solid state AVRs from Denon, Yamaha, Onkyo and the like but I want to decouple decodes and DACs from amplification so I can choose to optimally balance my amp & speakers as I have with the main L/R channels in the NestT.

So, I have my center channel and surrounds sources now but I need amplification and speakers. First though I'll do experiments with stuff I already own. I have amps and speakers and such which will not be perfect but will work well enough to see if I'm on a viable track.

Center Channel Experiments... First I tested the used gear I received for function and quality. I hooked the DP564 into my Mac and a DVD player and all works as good as new both from 2 channel PCM and 5.1 Dolby Digital sources.

Then I lashed the DP564 into my audio paths of the NestT, which actually did require some finagling. The miniDSP equalizer does not pass Dolby Digital so I needed to tap the fiber out of the TV, as it will send Dolby Digital when the material playing contains it; this required me to simply split the fiber coming out of the TV, sending one to the miniDSP and one to the DP564. After EQ the miniDSP will simultaneously drive the AES out and fiber out at the same time, so for the mains I'll drive with AES out through the DAC and for the CC, LS & RS I will drive with either the optical out of the miniDSP (2 channel PCM out used to create Pro Logic II derived feeds) or optical directly from the TV used to source Dolby Digital feeds, through the DP564. So I needed a remotely controllable fiber switch/selector in front of the DP564. The connections are straight forward but I am a bit concerned about processing latency getting something out of sync. I think I will be OK through as the Dolby box has configurable time delays; I'll assess during experiments. Analog out from the DP564 is balanced and I typically use unbalanced RCA for amp inputs (single ended triode, SET, amps are by definition unbalanced) ~ so I used some conversion cables too. Here's the connection stuff and an updated NestT Connection Diagram.

My original NestT gear rack is full so the Center & Surrounds expansion gear consisting of a DP564, amps and maybe a DVD player were first on top of the subwoofer; final installation will be somewhere else but I'll fashion a rack-like thing custom built to match with the other rack.

1st Experiment ~ I drove the awesome speakers which are integrated into the Sony TV itself as the system's center channel. I've wanted to try this since initially buying the TV and hearing it's native sound, but I never had the gear to do it. I used a simple T amp to drive the Sony speaker(s). I exercised the center channel both with YouTube material (below) and with JRiver flac material (above); each case implied the DP564 would derive a center channel for the Sony with Pro Logic II from 2 channel PCM. It sounded amazing for flac files ~ I was stoked for the new array even just using the TV speakers. Though when I played YouTube vids I encountered an intolerable processing delay between center channel and mains... I went on the hunt for schemes to better synchronize (more on latency later). Nonetheless, despite the encouraging flac playback as it sounded OK playing music ~ the front soundfield was still a bit disrupted in my mind. I believe this is less a fault of the CC derivation from Pro Logic II, but from the change in drivers which altered the timbre of the soundstage as it spread from left BLH to Sony center to right BLH. Good enough for movies but not critical music listening. Plus, setting up to always use the TV as a center channel would require disabling the TV's own Dolby processing for its own speaker array ~ which we quite like for movies. I will find the right delays I hope, and will use an external center channel speaker.

2nd Experiment ~ I used different center channel speaker(s) from NHT. I tried just one NHT to begin with on the T amp, but then also dropped in a second, since I had 2 channels on the amps anyway. The outcome here was more satisfying but I still felt there was a disruption in the force... I did not hear a coherent front stage through YT or movies but it was better on digital playback primarily due to using the miniDSP out to create the center channel; this eliminated most of the latency issues created when using TV out to create a center channel.

3rd Experiment ~  I "corrected" the NHT center channel using my miniDSP 2x4. My sense was that some of the delay and distinctiveness of the center channel in the soundstage was caused from the "uncorrected" nature of the center relative to the "equalized" mains (from their audio path through the Dirac Live miniDSP, which smooths frequency responses toward optimal in the NestT room ~ and also which must be slow as heck as delays are notable in measurement). This was painstaking as it required that I gauge in real time the delays incurred through processing and offset them in signaling to the CC. I also removed the split & switch fiber notion entirely as the time delay differences from using two differently processed sources to drive the center was not resolvable yet. I did not measure the NHT center channel SPL with frequency sweeps, but did use the miniDSP  2x4 to add CC delays till perfect. I will do the frequency sound pressure level adjustments when I've built the real CC. I also moved all my gear to the left rack in hopes of consolidating but it may yet find a different spot.

Measuring audio gear latency is a whole topic worth some focus so I will digress ~ at this point in experimentation I devised a larger measurement endeavor to determine sources and scales of audio delays within the NestT. That measurement study is HERE.

The NestT sits at 9800 ft elevation and assuming the temperature there is typically 65° F, the speed of sound (c) is ~1124ft/sec. So, to account for the 1 ft of difference in distance to the listening position between the mains and the center channel, I need to add just 1000*1/1124 = .89 msec delay to the center channel (not accounting for different processing delays). I went back to setting the center channel delay parameters in the miniDSP 2x4 and tried the calculated 12.2 msec (delay from DAC - delay from DPS64 + .89) from my latency measurements. I listened carefully again and sent some impulses through to distinguish differences ~ simultaneous arrival to listening position! Finally, on with the more fun stuff. Not so fast ~ the miniDSP 2x4HD distorts unexpectedly at reasonable listening volumes (at least in analog mode) and sometimes even looses it's mind on delay parameters. I removed it from my center channel chain but still need something to at least adjust time delays for equipment latency differences.

UPDATE 22Jun23... I did most of the delay adjustment inside the DPS564 but didn't quite have the range & flexibility I needed to achieve hifi. I again bought some old tech which fits the bill perfectly. I'm lucky to have found a Felston DD740. It's a competent, remotely controlled, digital audio delay circuit with 4 digital inputs and 2 digital outputs. In order to get unadulterated/unEQed stereo PCM signal into the DPS564 I split the the digital signals out of the phono A>D coax, Linux/DS10 streamer coax, and the TV fiber and routed to both the miniDSP DDRC22D (mains) and the DSP564 (center). I now get a well-derived center channel and can control the delay applied on any of these signals in the Felston box... and I even recall the setting for everything at boot up or on changing the sources, via a single remote controller. The connection diagram is updated as shown.

I am almost there... the Dolby Pro Logic II derived CC source is right and delays are adjusted to perfection, without distortion, now via DD740. By "right" I mean that the artificially derived center channel from Dolby Pro Logic II folds well into a front soundstage when volumes are balanced well. Dolby themselves explain: " PLII is an advanced matrixing technology which delivers convincing surrounds from stereo content; it uses hidden audio cues that already exist in stereo content to create realistic surround sound with five full-range channels." It is time to build a righteous CC speaker. It seems imperative for a center channel to integrate well that it must be built from a speaker array with pretty similar driver characteristics and behaviors to its' mains, or must be corrected to sound similar

UPDATE 26Jun23... The Felston DD740 has made all the difference to the center channel setup. I am able to use any of my normal sources to derive an associated center channel from the DP564, and I can recall an appropriate default delay for that audio chain. There is still however some dynamism in the delay encountered sitting to sitting and even track to track. Fortunately it is easy to adjust the delay I need using remote control of the DD740. I just wish I could automate, with precision, the measurement and delay tuning. Work for another day, as I don't have a solid idea how yet... probably mount a permanent measurement mic and use some impulse measurement to measure the delay between the speakers.