Cubox-i4 - SPDIF bandwith

I.MX6
1

If i check the wiki, it says:

The SPDIF/COAX port is limited to 15mbits - it will do 192kbps 2 channel PCM but won’t support any of the “HD” audio formats as there isn’t enough bandwidth - please use the HDMI port instead.

Wonder how the conclusion that it will just support up to 192kbps 2 channel PCM (or similar), if 192*2 = 384 kbps, about 3 mbit. There should be plenty for higher bit rates. Asking because want to use it with Spotify ta maximum quality, 320 kbit/s.

Perhaps my conversion is just wrong :). Thanks.

2

Doing some extra research, if i want to play at “CD quality” stereo, would be about 44hz x 16bit x 2, about 1.4 mbit/s. Well, under the max 15mbit/s.

Looks like the hdmi to RCA i just bought will not be necessary, and trashed 8 usd :smiley: (well, perhaps will be handy for other projects).

3

There are a few things going on here. First is that Spotify 320 kbit/s is a lossy codec, and as you found out generally 44hz x 16bit is plenty fine for the RAW audio link.

The “won’t support any of the HD audio formats” is in RAW bandwidth. DTS, Dolby Digital (newer) are all compressed audio codecs to support more channels over standard links. Generally SPDIF can support 24-bit audio but it is not supported so 20-bit is the standard bit depth. If you want you should be able to handle 96hz x 20bit x 2

Hope that clarifies things.

4

thank you for replying but the confusing part is, the port is limited to 15 megabits per second and then put the example of the PCM which says that it is 192 KB per second * 2 or whatever is the doable rate, implying that is the maximum it could get, but the truth is that there is plenty of bandwidth for higher bitrates left.

5

I think the wiki is incorrect. That should be 192kHz not kbps, and remember that every word has an 8-bit preamble. This make 192kHZ * 4 * 2 come in just over 12Mbps

6

192 * 4 * 2 * 8 is about 12MBps.

All this conversation made me paranoid abut the SPDIF port from the cubox. Is there any way for me to check the bandwidth used by it?

7

Where did you find the 15Mbps bandwidth limitation? The max bandwidth supported by SPDIF is 125Mbps, so just over 15MBps. However due to the clocks of the iMX6 can not full support this full bandwidth. The iMX6 can do 96/24 as I have tested this personally.

8

https://www.solid-run.com/wiki/doku.php?id=products:imx6:cubox-i

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9

BTW; I’m checking this schematics:
https://solidrun.atlassian.net/wiki/pages/viewpageattachments.action?pageId=197493654&preview=%2F197493654%2F244973632%2FCuBox-i%20Schematics

and looks like the output is a PLT133/T9 with limit of 16mbit/s. Perpahs thats why the wiki says 15mbit/s

Also uses the PLT133-T8, which is limited to 16mbit/s as well.

10

well that is per the spec. which is reasonable. The hardware can be pushed even up to 192/24 however it will introduce more jitter into the signal. I know from our hardware and other audio hardware projects that this part can do 96/24 with reasonable jitter. The data sheet and specs were also produced in 2005…so there wasn’t too much reason to validate hardware beyond 48/24 at that point.

11

and kind of an addendum. Even at Spotify’s 320kbit/s they are just using a higher bitrate ogg encoding and I believe they use a sample rate of 48/24 for it. So even at 48/24 you will not be losing anything from their original encoding.