Question re low rate audio sample rate over HDMI

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Joe_90
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Question re low rate audio sample rate over HDMI

Post by Joe_90 »

A poster on CoreElec forum has reported an issue with radio sound from the standard Kodi radio plugin (radio.de) on certain stations. He provided examples such as 2CH or 2GB or 2UE in Australia. I checked with my own N2+ and Sony DN-1060 AVR and have experienced the same loss of audio on those stations. I can see from the Kodi log that these are using a very low sample rate (22.05kHz). My normal Kodi audio settings are for "Best Match" and passthrough as my AVR will handle all standard sample rates and the usual high-res and standard PCM formats. However, when tuned to these particular radio stations I get no audio unless I change to "Fixed" output at 44.1 or 48kHz and let Kodi upsample. I can hear audio through the headphone socket on the N2 ok once I change the Kodi setting to standard PCM, so in in-built DAC is working ok.

Is there a restriction on low-rate PCM audio over the HDMI (and S/PDIF) outputs on the N2/N2+?

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Re: Question re low rate audio sample rate over HDMI

Post by rooted »

Have you verified this isn't a Kodi issue?

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Re: Question re low rate audio sample rate over HDMI

Post by Joe_90 »

I don't think it's Kodi as I have an S905X box which works fine on the same radio stations using the same plugin and version of CoreElec and the exact same Kodi passthrough settings. However, on this box the sample rate is forced upwards to 32kHz from 22.05kHz. Further investigation by me on the DN-1060 AVR has led me to think that the lower sample rate is not, in fact, supported by the AVR and that 32kHz may be the minimum rate supported. I can't fully verify this as the actual technical specs published by Sony are vague about the lower limit on HDMI but do state that network and USB file formats have a lower limit of 32kHz in certain formats.

Now, it may be that other chipset drivers may default to upsampling these very low-rate formats to a minimum of 32kHz because this is a common lower limit of many AVRs whereas the N2 actually does output at low rates and my AVR just can't handle it. Earlier versions of Kodi had a possible tweak in advancedsettings.xml to output a minimum sample rate (because some Pioneer AVRs wouldn't accept rates lower than 48kHz) but that's gone now.

I just wanted to verify that the N2 is actually capable of PCM output at 22.05kHz. I know the internal DAC is working ok as I can get audio from the headphone jack.

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Re: Question re low rate audio sample rate over HDMI

Post by cookieisland »

I am also experiencing this issue with an Odroid N2 and the Kodi radio addon. I have CoreElec 9.2.5 on an Odroid N2 connected via HDMI to a Denon 3808CI. I have Kodi audio 'output configuration' set to 'Best Match' and passthrough is enabled. Various radio stations will not play with the Kodi 'radio' add-on unless I change my Kodi setting from 'Best match' to 'fixed'. This does not happen on my previous Kodi box (an Intel NUC running ubuntu and Kodi). I used the same Kodi settings on the Intel NUC as I do on the N2. I have also tried to play a standalone mp3 test file with a 22.04kHz sampling rate and it will only output audio on HDMI if I switch the 'output configuration' to 'fixed' in Kodi which unfortunately disables passthrough. I do not want to disable passthough so it would be great if the N2 can deal with this like other boxes. This problem also occurred on previous CoreElec versions on the N2.

BTW: Here are a couple of example stations that were silent prior to turning on ‘fixed’
KGO-AM 810 (San Francisco bay area)
KQED Public Radio (San Francisco bay area)
2GB-873AM (tested this station due to a similar issue by another user)

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Re: Question re low rate audio sample rate over HDMI

Post by cookieisland »

BTW: Hopefully my message does not distract from the original question from @joe_90 with respect to whether or not the N2 is capable of 22.05kHz PCM output, I just wanted to add an additional data point because I am experiencing the same issue with my N2 as well.

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Re: Question re low rate audio sample rate over HDMI

Post by crashoverride »

Per the HDMI specification:
An HDMI Source is permitted to transmit L-PCM audio data at sample rates of 32kHz, 44.1kHz, 48kHz, 88.2kHz, 96kHz, 176.4kHz or 192kHz.
See footnote in https://en.wikipedia.org/wiki/HDMI

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