[mintzar]

I've been throwing around the idea of a fully digital multi-channel home theater. Doing away with the pre/pro, DACs, preamps, and all the conversion stages for something simpler and lower distortion.

Basically the idea is that I take the digital PCM stream from HDMI and decode it to multi-channel PCM outputs that would connect directly to my fully digital amplifiers. This, in theory, would allow for fully digital home theater. The amplifiers have a voltage controlled attenuator so volume control, in theory, wouldn't be an issue. And it might even be possible to build a multi-channel fully digital amp with HDMI input and HDMI output for video pass-through.

Am I missing something critical? Is that at all interesting to anyone?

[JP78]

I've been throwing around the idea of a fully digital multi-channel home theater. Doing away with the pre/pro, DACs, preamps, and all the conversion stages for something simpler and lower distortion.

Basically the idea is that I take the digital PCM stream from HDMI and decode it to multi-channel PCM outputs that would connect directly to my fully digital amplifiers.

I don't think this quite makes sense, as ears cannot hears in 0s and 1s but rather sound waves. Conversion has to happen at some point; you cannot do away with DACs. If you're doing about doing away with several boxes in an HT, then I think you'll find from your independent listening tests that $1500 street nowadays buys a very sophisticated, fairly good sounding receiver which sacrifices almost no functionality or ease-of-use.

And it might even be possible to build a multi-channel fully digital amp with HDMI input and HDMI output for video pass-through.

Am I missing something critical? Is that at all interesting to anyone?

Also, you may want to consider that a couple of the Blu-ray codecs do not support stripping the audio signal to PCM and must only be passed through HDMI, so your device(s) would basically have to work as an HDMI splitter since stripping audio off the signal may not be possible. I am not absolutely certain of this, so please independently verify.

[mintzar]

I think you might be misunderstanding the concept. Receivers are analog devices and, to many a discerning audiophile, are the antithesis of good sound. Digital on the other hand thrives on being processed in one location without handshaking or conversion. I would be doing away with the crappy converters, DSPs, and mediocre power supply in an analog receiver for a fully digital system. Where the signal is decoded and amplified in the digital domain.

There are several chips on the market that decode HDMI to i2S and then convert it to analog. I can take the i2S PCM signal before it's converted to analog. This PCM signal then can be reclocked and converted to a pulse width modulation where it is amplified in the digital domain and then converted to analog at the output of the amplifier.

This isn't designed to be a ultra functional receiver that sacrifices sound, but a fully digital receiver whose primary objective is sound quality. So I'm not just bypassing boxes and several noisy power supplies, but getting rid of several stages of conversion and poor amplification that home theater usually faces.

In a standard home theater you have a Blu-ray player. Let's assume you use HDMI out into a pre/pro. This pre/pre decodes the hdmi signal, converts it to analog, runs its dsp, and then sends these signals off to a separate amplifier, which amplifies in the analog domain or converts it again from analog to a PWM signal to amplify it. With a receiver it's even worse where it converts and amplifies all from the same noisy circuit. I'm talking about taking the hdmi signal from the bluray directly into a multichannel digital interface. Amplifying it in the digital domain and having only one stage of conversion at the output of the system. Basically combining the amp and pre/pro into one box and handling everything in the digital domain rather than the analog domain.

[ted_b]

Ryan,
Hi.  Long time no talk.  I would love to chat about an all-digital home theater.  As you know I've invested heavily in both 5.1 mch (for music) and 7.1 mch for movies, along with my true love, 2 channel music. (see my system link)  I've also helped Mr Wicked and his folks pioneer DSD ripping from SACD's, and have become enamored with that format (along with my push for all things hirez).  These ISO's are then extracted to stereo and/or multichannel pure DSD (DSF or DIFF).  So my request would be to get DSD processing in your plans as well.  My current multichannel approach is stagnant for the moment; it is still disc-based cuz although I have a large and continually growing computer-based mch library I can't decide on what is the optimal multichannel software player (very few), OS and DAC (likely a DSD-capable one like Mytek...my Mytek 2 channel DAC does DSD very very well)

I have unique concerns about the design due to my separate dedicated 5.1 (full ITU identical setup) and 7.1 (uses different sides and rears).  Things like presets that account for different channel trims for both, slightly different bass management for each, and of course routing to different speakers for 5.1 vs 7.1 (shown as red and green arrows below)  Today that is handled in my mid-fi Denon 38xx receiver that I use as a mch pre-pro.  My 5.1 hi-end multichannel hirez music was much better presented when I bypassed all this for that function, and went all-analog mch pre...but the complexity, redundant cabling and upkeep was too much as I attempted to also demo other ideas.





My point is that simplification is key to fidelity, especially as we discuss the digital signal path.  However, I am quite happy with my main Modwright monoblocks for 2 channel (and front l/r), so the tradeoffs on an all-Ryan theater need to be weighed. 
Ted

[mintzar]

I'm totally with you there, Ted.

I think DSD is the way of the future for sure. My amplifier already processes a 1-bit PWM signal (which is what DSD is basically, but at a lower sampling rate), so I'm working on implementing that technology in near-future iterations of my amp.

I can build a fully digital home theater. There are several HDMI demux chips that take HDMI, decode it to i2S and then out analog, but tapping off the i2S multichannel signals isn't a problem -- I think there was a clux product that did that for a while. I can then take the i2s directly into my amp... it's really not too complicated a setup.

I think it's the future of multichannel audio... especially if I build an entry-level consumer version it can be much cheaper than most receivers, but still maintain the versatility and higher performance with a high-end version that is truly high performance. After going to a fully digital signal path it'll be VERY hard to convince me that there's a way of doing anything better when digital is concerned -- digital thrives when processed in one location.

Granted, I'm working on a discrete version of the amp too using an FPGA, which I'd also like to implement in this fully digital home theater concept.


That's a great lookin' room! I currently have my home theater set up with 4 of the same speaker, I haven't pursued more channels yet... because I kind of like the idea of 7 of the same crossoverless speaker and I've been busy playing with other things.

[wisnon]

I think you might be misunderstanding the concept. Receivers are analog devices and, to many a discerning audiophile, are the antithesis of good sound. Digital on the other hand thrives on being processed in one location without handshaking or conversion. I would be doing away with the crappy converters, DSPs, and mediocre power supply in an analog receiver for a fully digital system. Where the signal is decoded and amplified in the digital domain.

I used to have a Class D "digital" amp but it used Pulse WIDTH Modulation, not PCM. It also only accepted analog input and output was also analog. I know the NAD master's series M2 accepets digital input, but I think its also uses PWM amp technology. If so, it would either not accept PCM or if it did, it would convert to PWM (ie DDC). That defeats the purpose somewhat.

Not sure which quality digital amp uses PCM and also accepts PCM input.

Edit, it seems the M2 may work well (for 2 channel though):
"Contrary to usual preference, I’ll segue into this review with some verbal definitions. Most of you already know that class D stands for neither class Digital nor means that this class of operation is digital per se. While the basic operation of this very energy-efficient approach does resemble digital by consisting of just two switch modes of the output devices—on and off like 1 and 0—the more common pulse width modulation technique is actually defined by the duration between switch changes. And in theory, this duration—very non-digital at that—can assume endless intermediate values to not be limited to given samples. "PWM is definitely analog" is how Nelson Pass once put it succinctly.

 
This hasn’t prevented many from equating class D with digital. What confuses matters further is that class D amps are usually fed analog signals even though a few exceptions are driven by digital signals. In the latter case, PCM signal of digital sources like CD or network players connects directly to the amplifier input. Analog sources then have to undergo A/D conversion the moment they enter the amp. This gets us to the NAD M2. It belongs to the still rarer species of digitally driven class D amplifier which process the signal in the digital domain. After filtering out the switching frequency components, the outputs of course pass analog signal. In that sense, the M2 is a ‘true’ digital amplifier.

For advantages, NAD cites "the elimination of analog distortion, noise and hum in the preamplification and power processing stages" and also describes the M2 as fundamentally an over-dimensioned D/A converter (at 200wpc into 8 ohms the M2 is stoutly potent even for an integrated amplifier) which has practical consequences.


The concept begs for a bevy of digital inputs and NAD offers five: 2 x coax, 2 x optical and 1 x AES/EBU all of which accept 16/44.1 to 24/192 signal. There are also coaxial and Toslink digital outputs. The M2 further accepts analog inputs on RCA or XLR which, naturally, encounter immediate A/D conversion via TI PCM4222 chips that are selectably upsampled to 48, 96 or 192kHz but always at a fixed 24-bit word length. This creates an additional format conversion layer and NAD admits that the digital-direct inputs have a sonic advantage over going in analog.

While digital signal is processed up to 24 bits and analog automatically converted to 24 bits, NAD actually talks of 35-bit math. At first glance this makes no sense. It's simply about mathematical headroom however. Envision a basic pocket calculator. If it accepts 24-digit figures, subsequent calculations can easily exceed those digits. A calculator with 35 digits will produce more accurate results.

This internal processor density also ties to NAD’s DDFA™ direct digital feedback amplifier scheme which the development team seems particularly proud of. It allows additional corrective math just ahead of the speaker outputs. In collaboration with Manchester-based semiconductor specialist Zetex, NAD has developed a circuit topology which "corrects even the smallest pulse deviation" by comparing the PCM input signal against an internally generated PWM reference signal. Possible errors include pulse amplitude (incurred by remaining power supply rippled), pulse width and a softening of the pulse edges – all factors with are directly relevant to the quality of the music signal."
 

[JohnR]

I would love to have an "all digital" HT. My understanding is that decoding the 5.1 entails significant licensing fees, which is only affordable by fairly large companies. So what I do is feed 5.1 analog into A/D convertors, you know the rest.

[ted_b]

I would love to have an "all digital" HT. My understanding is that decoding the 5.1 entails significant licensing fees, which is only affordable by fairly large companies. So what I do is feed 5.1 analog into A/D convertors, you know the rest.

Or do the decoding in the player, then send 5.1 hirez LPCM (still digital).

[mintzar]

Right, I send LPCM out of my devices anyway and really don't hear much difference between the source doing the decode vs a pre/pro doing the decode. So it's of little detriment to take the pre decoded LPCM signal and break it down into its subsequent i2S components.

But that's only a short term. I'm working to arrange the licensing so that it can be built as a real product and not just a short term DIY-esque solution.

Time permitting I'm going to build one in the next few weeks and see if it even works!

[ajzepp]

Ted: I never get tired of seeing the pics of your room and set-up...one of the best I've ever seen 

[ted_b]

Ted: I never get tired of seeing the pics of your room and set-up...one of the best I've ever seen 

Thank you.  What a nice thing to say.  I am proud of it, especially how well it does at being a great 2 channel room without losing any multichannel benefits.  C'mon Ryan, bring on the all-digital stuff.   

[ajzepp]

Thank you.  What a nice thing to say.  I am proud of it, especially how well it does at being a great 2 channel room without losing any multichannel benefits.  C'mon Ryan, bring on the all-digital stuff.   

That's why I like it so much...I love hybrid systems, and that room is amazing

[DS-21]

If you're doing it for yourself...sounds cool. Especially if one of those stages is a DSP that gives you fine control over crossover slopes, parametric EQ, delays, and so on. If you're not planning that, see below.

Panasonic did just what you want. They had a line of receivers that were "digital to the binding posts," using TI's PurePath PCM->PWM power-DAC chipamp. (There was an A/D converter for the analog inputs.) Many of us bought and used them. I would still be using a successor product to my old XR55 had Panasonic stuck with it and introduced one with a good room correction system.