HDMI audio

OK, thanks.

The key question for me is how anyone would de-embed to spdif >48k LPCM stereo when their monitor can’t accept it.

The concept of the expensive surround theatre amps must be to intercept the audio from the hdmi and pass the hdmi on to the display device in this way? I realise there are different versions/capabilities of interconnecting with hdmi but that would have to be fairly necessary I would think. The advantage with a surround amp you could decode dolby/dts surround and the later 7 channels etc I suppose, and possibly the main ‘theatre’ application, however the dacs for stereo are likely to be pretty good.

The problem with HT amps/receivers is that they cost a lot and do many things I don’t want! Nor will most hi-fi stereo fans. The basic requirement here is to extract spdif from a RO machine so it can be played with a decent audio DAC.

Personally, I think the way most audio/hi-fi people would choose is USB. If we had that, this requirement would become less important. But since that still hasn’t been integrated so it can be used via SoundDMA – whilst HDMI has been integrated – it makes sense for people to be able to use it.

Some monitors have spdif outputs. But all in my experience regurgitate everything as 48k – even 44.1k. So are handicapped for serious high quality. Such monitors may degrade even Audio CD material!

BTW Yes, I do have in the living room a unit that can be used as a ‘home theater’ system that can accept audio over HDMI and do a decent job. But it cost a lot more than a decent stereo DAC and is much bigger. My concern here is for people who want decent stereo audio and don’t want to be driven to a HT box which has many features of no interest to them because it is the audio they’re after.

I realise this is a minority sport, but so is using RO. And as it is, the lack of integration of USB Audio with SoundDMA puts RO behind the main OSs people use. HDMI now provides an alternative when we can identify some suitable de-embedding devices.

Hence the need for a box that will de-embed spdif even when the monitor can’t cope with it.

The box should be here later today.

I think, being honest, most RISC OS folks would be more interested in the analogue outputs from the box (to hook up nice stereo powered speakers that they already own) rather than SPDIF, since you need a DAC/decoder/amp/soundbar to handle the SPDIF digital output.

I suspect a majority of people won’t actually understand what SPDIF is all about, nor what a DAC is, so a lot of this is probably wasted. However, I think that for people using HDMI, there might well be a desire to split off HDMI audio to (say) speakers or their hi fi, which the box will do nicely.

We’ll see.

I’d agree that for many a ‘breakout box’ that delivers analogue would be preferred. For me the advantage here of one that also outputs spdif it is makes it easier to validate what the box is doing. e.g. seeing the spdif output 44k when fed 44k helps in making it seem more likely that is what’s being converted to analogue. And of course, the spdif then comes in handy when some may want to connect to other kit later on.

I’ve been told the de-embedder/switch I bought second-hand was posted this monrning. So all being well I can test it in a few days and then report on it and decide what to do next.

Not sure I’d 100% agree with Andrew, it depends on how technology is presented.
There is quite a bit of equipment with optical audio now, you can pick up a cheap soundbar for not much now with optical audio, so I’d suspect most HiFi’s should have them now? (again I have no knowledge of this, it’s a wild assumption)
Even when I’m on the PC, I play sound on my MX6.
I’m slightly confused why no-one else has tried this, the HDMI sound quality is awesome. Maybe it’s just the sound on my PC is rubbish, that is entirely possible.

If you say SPDIF and DAC people may get confused, but if you say optical sound and USB sound output – whilst possibly missing the technical details, it’s not as bad as the “coffee cup holder”.

One thing that is irritating is that games tend to stuff the sound – usually when you quit, sound don’t work™

At the WROCC last night I managed to demo the sound working, the “converter” was connected to another box which connected to the projector and my monitor.
It responded with the projectors ID and discovered the modes and the sound worked (well after a few minutes wondering why it wasn’t working as the optical input wasn’t selected on the soundbar)

One thing that would be nice would be an edid → modefile app. I have a feeling I’ve seen one somewhere….

Some USB audio devices have limits to what their spdif will output/input, It will of course be useful to know what your convertors can actually do. I imagine high end dacs would put the price of a convertor up considerably, then you would have to establish just what quality you are getting for your money compared to a good dac.

I can give an update to this:

I got my second-hand ‘Monoprice’ HDX-401TA this morning and had a brief experiment with it. I’m using it between my ARMX6 and a IPS235 monitor which itself reports only supporting up to 48k sample rate for LPCM audio.

With the HDX in the path !SoundCheck reports all rates and sizes up to 192k/24bit. I connected via optical to my DAC Magic plus and when I play a 92k/24 file using DCD this plays out via the DAC with its 96k led lit.

So by default, this works as a way of getting LPCM out of the ARMX6.

A 192k test file also ‘almost works’. DCD plays (I can see the PPM levels waggle, etc) and SoundCheck says the system rate is now 192k but no led or sound on the DAC Magic. I suspect this is because optical can struggle with 192k. So I will try coax at some point. However it may be that this rate is too much for the setup as I can then hear audio from another pair of headphones on the analogue output of the monitor. (Which downsamples everything to 48k.)

So far, fairly promising. But not doubt bugs lurk.

Oh BTW the switch eats 440 mV at 5V. Mention that as it comes with a 5V/2000mV rated wall wart, but this has a merkin plug so I’m using a bench PSU until I can bodge a PSU together.

The instruction ‘manual’ (sic) is, erm, ‘interesting but vague’…

Jim

I can add a bit to the above:

When I tried a coax spdif from the above de-embed device to my DAC the 192k did work… sort of. The 192k led lit and I could hear the music. But it kept having gaps and clicking. If I stopped DCD playing it, I still got clicks rather than silence.

So either the de-embed box or my ARMX6 couldn’t keep up with 192k. 96k seems fine, though. I’m listening to a 96k recording (Benjamin Britten Sea Interludes) as I type this.

I’ve now tested what the device/manual calls a 2ch/5.1 switch. By default the switch/de-embed box comes on with a red led lit that is supposed to signify ‘5.1’. Yet stereo LPCM does work, and at 96k.

If I switch to ‘2ch’ the list of modes falls back to those offerred by my monitor and I don’t get anything out via the de-embed at 96k.

So I suspect this switch really flips between allowing the monitor to rule or having the switch rip out a copy for formats above what the monitor says it can handle. I think the switch may also help the monitor here and pass on a downsampled stream. But it may be the monitor just reads and skips.

A quirk is that the device in its ‘5.1’ mode says all the LPCM is ‘channels 8’. but it doesn’t seem to have a problem passing stereo on to the DAC. It also lists AC-3, etc, but I don’t have anything to test that with.

The Britten sounds quite good. :-)

Jim

Jim, like most things, you may have to try the convertor from another platform to see what its capabilities are.
I think ‘up to 192K’ is sometimes used for advertising in usb audio but can mean recording a mono channel. It may be a USB 1 limitation.
There are newer formats for surround that can utilise 192K spdif, the only one I have had experience with is the 5.1 dolby compressed and padded into a 16bit 44100 Wav container. There is info in the book by the american author who was an editor in a hifi magazine. Hartley? Time to get the book out again anyway. ‘channels 8’ could be the 8 channels for 7.1 surround?

@Jim – That’s really interesting.
Out of interest, you would you rate the output quality?
Does optical make a difference (as it should be a straight digital→digital copy, rather than coax which will be digital→analogue) – or is there more to it than that!
Be good to get feedback from someone with a good appreciation of sound.

I can’t yet comment on quality in any detail. Not had enough time to test. However I suspect that – within the 16 bit SoundDMA limitation – the results should be bit perfect if you playout using DCD or other apps with the option to bypass any software/system volume controls. (Using these may degrade the results.)

How accurate the timing, etc, may be, or if some samples are getting ‘dropped’ or ‘repeated’ will take more testing. But if the de-embed is working the results should end up depending on your choice of DAC. The device I’m using does have an analogue output but as yet I’ve not tried it. One step at a time…

The main advantage of optical is that it breaks any potential ground loops or RFI injection from source to DAC. But optical tends to struggle to work above 96k. However that probably won’t matter to most people. TBH I only have a ‘technical’ interest in > 96k as 96k sounds just fine to me in general!

Does optical make a difference (as it should be a straight digital→digital copy, rather than coax which will be digital→analogue)

I am sure the “coax” Jim refers to is also digital→digital, just in an electric instead of an optic way for signal transfer.

Yes, sorry. I didn’t detect that.

Yes, the ‘co-ax’ I was referring to is spdif over a 75 Ohm co-ax not an analogue pair.

For those who may be unaware. For some time it was common for ‘optical’ digital connections used in home audio to not be able to carry the highest data rates. So although a device might have optical and co-ax digital sockets, the optical one might not work when you try rates above 48k, or these days > 96k when the electrical co-ax connection does work.

This is less common now as the cables, modulators, and detectors have improved – at least in good kit. So these days good audio kit should work OK either way. But not all optical ‘cables’ are up to it. The home use optical ‘fibres’ used for this are nothing like the ones used for pro communications. So a long or cheap one may not work well.

FWIW For best performance a co-ax spdif digital link like might be preferred for use with high data rates like 192k/24bit. But they do risk problems with ground loops or RFI which optical connections block. So in practice being able to experiment and compare can be useful.

Thanks for that Jim, nice for the “unwashed masses” (i.e. me) to be educated.
My experience of optical cable is purely within an IT perspective.
I had no idea you could (or would want to) use co-ax cable for a digital signal when optical is avaliable – but optical is still massively expensive for decent stuff.
I always assumed SPDIF was always optical.

Clearly I have a lot to learn, which is a good thing.
I think I’m going to have to “sneak” some audio equipment in the house slowly and do a lot of research first.
As I’ve just bought a soundbar, I’d get into big trouble if I ordered some separates and speakers right now. Sometimes (most of the time) I hate having to be sensible :-(

I’m a bit puzzled by your comment about optical being expensive. It seems common on standard flatscreen TVs, etc. Not at all expensive.

I’d have guessed that many soundbars for use with a TV have an optical input. But I don’t use a soundbar. I use a DAC and a stereo system with two speakers. (OK, Quad ESLs. 8-])

SPDIF was originally entirely designed for use over electrical coax in the domestic situation for use with Audio CD. The optical method then came along as it avoids things like ground loops, etc. As time passed the specs got upgraded to cope with higher data rates for things like high sample rate stereo and surround ‘bitstreams’. (The switch I have says it will take surround streams, but that’s not something I can currently test. I’m not sure if SoundDMA can even be tricked into passing it!)

The switchbox claims to accept 8 channel LPCM at 192k. But I have no idea how I’d test that. In practice, though, I don’t personally have any interest in surround sound.

The home use optical ‘fibres’ used for this are nothing like the ones used for pro communications. So a long or cheap one may not work well.

Interesting that the common SPDIF cables seem to be cheap plastic fibre and have prices that exceed the multifibre (glass) types used in data comms. Typical OM3 “multimode” is less than a fiver for 1m SC-SC (the sort of thing I have in small piles in several comms rooms)
Typical transmission distances for the 850nm 1Gb/s optics is 550 metres¹ the SPDIF standard seems to be using 650nm and managing 10 metres max, which is probably down to the poor spec. of the fibre.

¹ No I don’t have patch cables like that. Those are infrastructure cabling runs – 497m longest, the longer stuff is single-mode which is good for kilometres.
Note: This editor can’t spell metres (probably wants the incorrect US spelling)

Yeah, but you can buy those “special” pro HDMI cables for 80 quid….cough…
Also there is the risk of death from glass fibre (when splicing and getting glass wandering down your blood vessels along to your heart), possibly why it’s not avaliable at home? Unless that is an urban mith that we get told to make sure we wear gloves.

Oh make sure you get a gold plated optical SPDIF…
say what???? I guess it makes the light move more smoothly.

Also there is the risk of death from glass fibre (when splicing and getting glass wandering down your blood vessels along to your heart),

Never heard that particular oblocks. Have any of these myth merchants ever seen a fibre splicing session? More to the point has anyone here tried to even cut a standard fibre patch lead?¹ Using standard side cutters you can cut the exterior and break the fibre in the centre but dealing with the protective layer is a difficult task. It’s like something akin to kevlar in it’s robustness.

Oh make sure you get a gold plated optical SPDIF…
say what???? I guess it makes the light move more smoothly.

Pixie dust² embedded in the gold helps the light slide into the fibre and down the length.

¹ Not a routine thing because I try and avoid damage but I like old leads decommissioned to be visually notable as non-functional. Hence UTP patch leads get a dose of the old side cutters.

http://message.snopes.com/showthread.php?t=27712

Ah Pixie dust, there are three kinds I use:
1. Blue Pixie dust – this is for networking
2. Green Pixie dust – this is for infrastructure
3. Red Pixie dust – this is used for servers

Any ideas the colour of audio pixie dust?
(from that you may deduce the level of sarcasm I tend to unload on people who I deem fit for it)

@Clive/Steve – Thanks for that, I’d assumed it may be an Urban Myth to give health and safety something to say.
Next they will be saying we can’t use angle grinders to remove those network cables stuck in servers.

@Steve – No, I bin ‘em we have enough patch cables. Never tried to open one TBH, I’ll give it a go next week.
Amazing what some people do with fibre before they connect it, leave it dangling on the floor etc.
grrr……thankfully everyone doing it at the moment has been well trained.

Interesting that the common SPDIF cables seem to be cheap plastic fibre and have prices that exceed the multifibre (glass) types used in data comms. Typical OM3 “multimode” is less than a fiver for 1m SC-SC

Not sure what you mean. Standard TOSLINK 1m costs around 1.20€, 5m costs 3.20€.

Of course there are those “high end” cables which are especially senseless when talking about such simple digital optical connections. But I guess you also don’t buy your ethernet cables from Denon, who managed to sell the “Ultra Premium AK-DL1” in 2008 for nothing less than 500 US$. Ah well, it was 1.5m long after all. Produced some great Amazon reviews however, which again shows that everything is good for something.

FWIW I got various lengths of brands and types of the audio ‘optical cables’ some years ago. Some of them – particularly the longer ones – visibly reduce the brightness of the light over just a few meters. Some work for higher rates than others. So the lack of real minimum specs here is a pest. But some cheap ones worked fine.

I got similar results when I tried various HDMI cables. Most work, but some give problems. The specs at the ‘transmission line’ level for HDMI are, erm, crap.

This is a shame as in both cases it can end up being used by those who want to flog ‘super’ cables. But its consumer free market, innit… :-/

BTW as I type this I’m listening to a 44k1/16 flac file being played via

DigitalCD → SoundDMA → HDMI → breakout box → optical cable → DAC Magic → Armstrong 626 → Pair of LS3/5As.

Currently some Jazz. Sounds OK. :-)

Just wondering if there are any updates or tips on this?