Insights into aliexpress DTS decoders, with some measurements

[MCH]

Hi,
I have spent some time looking into these cheapo black boxes from Aliexpress that claim to decode DTS AC3 etc from toslink, and give you 5.1 audio without using any AVR or similar. I am talking about these:

The objective for me is to build a very small (needs to fit inside the enclosure of my existing camilladsp rig that is already small) and very cheap solution to be able to get 5.1 audio when i watch a film in Netflix or Amazon Prime. I don't care much about excellent sound quality, and less now that i know that what most of the channels output is just noise and crickets, but needs to be possible to integrate in my current raspberry pi/camilladsp system that does room correction etc. and that i use for stereo music listening.

Looking at teardowns of these devices, one can see that some of them expose the i2s signals and use 3 stereo DACs to output the 5.1 channels. This is perfect to tap the i2s and bring it to a new Raspberry pi 5, that accepts up to 8 channels i2s. Some of these are very cheap and take a multichannel SPDIF signal (not HDMI) and do the decoding and DA conversion:

However, it seems these boards are old models now unobtanium. No matter what the picture is, or what the seller says, if you order one of those you will get one slightly different that does the digital to analog conversion in the main chip and does not expose the i2s signals. I blind ordered two times and got two of those:

Anyways, as i had them, I had a look at what they can do.
The good news is that they do what they claim. Plug the toslink out of your tv, select 5.1 audio and this is what you get.
Interestingly, the green one has a nice feature. It applies a high pass filter to the mains output and a low pass filter to the sub, and this is always active, also in stereo mode:

*NOTE: i don't have a way to measure in 5.1 mode, all the measurements in this post are in stereo mode.

But the performance of the internal DAC is very bad. And when i say bad, i mean very very bad, what is a shame.

The black one is more interesting though. The sub out is always active and low passed, but the mains are not high passed. The DAC performance is a lot better though, and i was able to measure SINAD of -72dB in my sketchy measurement setup, that might be even usable for explosions and traffic noise:

This measurement is from a 1 kHz tone generated in REW -> motu ultralite mk5 toslink output -> decoder toslink input -> decoder analog out -> motu line in with one of the balanced pins shorted to ground.

However, i really wanted to be able to tap the i2s signal and finally have an excuse to buy a raspberry pi 5 for Christmas. So i ordered another very similar device that i was confident had the i2s signals exposed. This one is a bit more expensive (i paid 36 euros) but as a bonus also takes HDMI ARC. Well it also takes bluetooth and memory sticks but i am not interested in that. The boxes look almost identical save for the additional inputs.
Inside you can see the three DACs (Everest Semi ES7154) that are a knock off of Cirrus Logic CS4354, pin compatible too. Other than that the main unmarked IC does everything, save for the bluetooth input. The HDMI input only takes the ARC related pins. The three buttons are to select the source (seems to have an auto mode) and volume +/-, unfortunately the volume of the tv does not have effect when using the ARC input:

Like the other boards, it does what it claims, both via TOSLINK and HDMI ARC. Additionally, they work from power on without having to press one single button, what makes them ideal for my use case. The performance is what you would expect from the CS4354.

But so far i had measured only from signals coming from my Motu interface, but how are they going to perform when connected to the TV? I didn't even try to find a 5.1 test tone and finding a way to output a decent stereo test tone out of my tv was more challenging than expected. All the files i found in youtube and spotify were crap and i couldn't stream a file to my tv. But i found a website that has a tone generator that works surprisingly well. ( https://www.szynalski.com/tone-generator/ ). With this website running in my tv, toslink to the Motu, this is what i got:

There is quite some noise that makes the sinad fall to -91 dB but good enough for my purposes.

But here is where things started to get complicated. Doing tv -> toslink -> decoder boards -> analog out -> Motu there was no way to get a decent measurement. I have no idea what the problem is, both boards without tv involved and tv without board involded behaved decently, but when adding the board between the TV and the interface, the performance goes south.

I am convinced that it is something related to my measurement setup, a ground loop or similar, but i am not knowledgeable enough to troubleshoot it. So i decided to do what i first planned for, to tap the i2s signals of the third board and go all the way in the digital domain.

I will explain this in part 2, hold on there.

 

[MCH]

Part 2.
As i was struggling with the DA conversion, i went ahead and desoldered one of the DACs of the decoder and connected the front LR i2s signal to a i2s to SPDIF board that i built for a different project:

The decoder board is the master. Sample rate is 48kHz, i wonder though if what you input is a different sample rate it outputs the same or resamples. I will test that another day.
No surprises, the digital transfer working straight away. However, i hit an issue that i was not expecting, due to my ignorance:
The Motu Ultralite mk5 that i am using to run the measurements, can be set to use its internal clock, or the clock from the toslink input. When i try to measure in loop (Motu toslink out > decoder > i2s to toslink board > motu toslink in), it cannot lock to the toslink clock because the setup does not generate a clock when the motu is not sending a signal. When using the internal clock, the measurement is very messy.

But i was assuming that when i go and measure the real case setup (tv toslink > decoder > i2s to toslink > motu), then the motu would be capable of locking to the incoming toslink, but no... I have no idea what is the problem here, if someone can advice, i would appreciate it.
The only way to overcome this issue was to use a minidsp DDRC24 as a capture card, so the setup is:

tv toslink > decoder > i2s to toslink board > toslink input of DDRC24

This way i could get a decent measurement:

Not bad, remember that this is from a website spitting a sine wave through my tv and then all that convoluted measurement setup with sketchy provisional solder points and no decent ground return path.

This demonstrates to me that the concept is usable and that the decoder board does not mess too much the signal. But all this is regular stereo, now i would like to measure an actual 5.1 signal, is anyone aware of any website that has a 5.1 sine wave generator or some sort of test files?

In the meantime, i will order a rpi 5 and attempt the definitive step: bring the digital 5.1 output to the rpi then do room correction and use it in an actual 5.1 system. I am not in a hurry because i don't even have a center speaker, and the other thing i learnt during these tests is that most of the dialogue goes through the center speaker, so no way to watch a film without....

 

[mcdn]

Wouldn’t the lack of a centre speaker be an ideal test? You can use camilladsp to route the centre signal to the front and right speakers

 

[MCH]

You are right, I had not though about it.

Actually the center channel is what is going to be more difficult to set up. We are moving in a few months and chances are that the TV will be a wall tv. A speaker just beneath is going to look awful. I don’t know how people solve this. The pictures I find in the internet look really very ugly.

But no center channel kind of kills the purpose of 5.1. Maybe I can start low profile mixing the central channel to LR with camilladsp as you say and wait for the occasion to get approval from the family… it is going to be a tough one, even for me to accept it.

On a different subject: I forgot to add it, and even to capture it, but initial testing shows that ARC performs the same as toslink, what is ideal for my use case because I can keep using the toslink for stereo listening as I am doing now and connect the decoder to ARC permanently.
It is just such a bummer that volume control from TV doesn't work.... I need to look more into this, maybe it is hackeable

 

[restorer-john]

But no center channel kind of kills the purpose of 5.1.

Yes and no.

5.1 is discrete, but the equivalent of phantom can be easily achieved by simply splitting the centre channel signal from the decoder and mixing it passively into both the L and R. With no centre speaker, you still get all the important discrete channels and chit flying around your room. Also, you don't have to have some ridiculous, ugly and compromised centre speaker messing up your room.

I haven't used a centre speaker for many years. They are totally unnecessary if you have a small to medium room, not many listening positions (1 sofa) and excellent imaging speakers. I can sit way off to the side, almost outside the "zone" and the dialogue appears to come from the screen.

 

[MCH]

Yes and no.

5.1 is discrete, but the equivalent of phantom can be easily achieved by simply splitting the centre channel signal from the decoder and mixing it passively into both the L and R. With no centre speaker, you still get all the important discrete channels and chit flying around your room. Also, you don't have to have some ridiculous, ugly and compromised centre speaker messing up your room.

I haven't used a centre speaker for many years. They are totally unnecessary if you have a small to medium room, not many listening positions (1 sofa) and excellent imaging speakers. I can sit way off to the side, almost outside the "zone" and the dialogue appears to come from the screen.

That's encouraging. I will try that, and maybe stay 4.1 forever, will make things so much easier.

 

[mcdn]

That's encouraging. I will try that, and maybe stay 4.1 forever, will make things so much easier.

I agree with @restorer-john . People think of centre speakers as “dialog” speakers but they are absolutely not. They usually carry all the music and main effects as well, turning your nice fancy L&Rs into glorified satellite speakers. Which makes sense when you think about a cinema where the “centre channel” is the width of the screen! So yeah, if you want a centre make sure it is at least as good as if not better then your left and right.