ATSC 3.0 Slow channel changes and wrong audio sample rate

Beta Community Apple Beta
#1

Just got ATSC 3.0 signals here on Monday. I have Channels (beta) running on 3 ATV's. I am seeing that changing channels between the 3.0 signals is very slow. Understood HEVC takes longer to build the stream. BUT I also have HDHomerun apps installed on the ATV's, and on the LG OLED and the OLED also has a built in IPTV app of its own. Changing 3.0 stations on HDHomerun or the built in app is much much faster than on Channels. On Channels, video stabilizes after 6 seconds, audio after 8 seconds.

Also another possible bug, When watching 3.0 feeds the audio sample rate is reporting at 46034hz. The chief engineer at the station reports they are encoding at 48000hz which is the standard. Not sure if the decoder is malfunctioning or stats is just improperly reporting the sample rate.

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#2

Submit diagnostics after slow channel change

#3

Ok, I will get that to you soon.

#4

Just sent the diagnostics

#6

Please hit record on your ATSC3 station, wait a minute, stop the recording, then email the mpg file to [email protected]

#7

clip on its way to you now

#8

For me, it usually only takes about a second longer than ATSC 1.0 to tune an ATSC 3.0 channel, but sometimes it takes longer. This seems kind of intermittent - I just had a channel take 7-8 seconds the first few times I tuned to it, but a few minutes later (with the same program airing) it took only 3-4 seconds. It is taking 7-8 seconds again, so I’ll go ahead and send a capture (though I’m not sure how good it will be given how intermittent this is).

One note - for me there is one ATSC 3.0 channel that always takes 7-8 seconds to tune, but upon looking at the HDHomeRun status it seems that channel is broken and Channels is falling back to ATSC 1.0. So you may want to first make sure that isn’t happening…

1 Like
#11

Spoke to a friend who is running a similar setup and he is also seeing the audio sample rate reported as 46034hz.

#12

Had my friend run TS Reader on the 3.0 stations and it also reported the sample rate as wrong, so it’s not just a bad stat report. The question is, is this a decode issue in Channels or a problem with HDHR tuner? Chief engineer at station swears he is encoding at 48000.

#13

The sample rate is being displayed incorrectly. It is not related to the slow tuning issue.

#14

Yes, not correlating slow tune with audio sample rate. Separate issues. But since TS Reader and the internal stats tool in Channels both report an incorrect rate it seems like that rules out at least the part of Channels that displays the stats. Also, as TS Reader is getting its stream from the output of the Channels app it still leaves me wondering where the error is being introduced. He uses the same HDHR tuner I am using.

#15

Have you tried recording directly from the tuner, either using hdhomerun_config or an HTTP request with a duration query parameter? That way you can see exactly what the HDHR is sending out, and eliminate Channels from the equation.

#16

46034hz turns out to be an interesting number. Doing some research and remembering my days as a video editor, 46034hz is the resample rate you would apply to 25fps video being converted to 29.97fps in order to keep the audio in sync with the converted video. I have no idea how that applies here since the frame rate of the station is 59.94 1080p, but it’s not just some random number. It’s also part of the ITU standard

34 Rec. ITU-R BS.1196-7
AC-4 also supports three bit allocation modes: Constant bit rate, Average bit rate and Variable bit rate.
The AC-4 encoder may be implemented with a variety of coding tools, shown in Fig. 15, to improve efficiency and flexibility/functionality depending on the mode of operations and/or application. The order of how the tools are implemented follows from left to right, meaning that PCM audio would be input to the tools on the left first and an AC-4 encoded bitstream would be output on the right. A description of the coding tools are provided below:
• SRC: The Sample Rate Converter tool is required in the AC-4 encoder to enable a frame duration that matches a video frame for frame synchronous coding modes. Depending on the frame rate, the input signal is converted to one of the internal sample rates of 46,034 Hz, 46,080 Hz, 48,000 Hz or 51,200 Hz used by the subsequent QMF and MDCT-based encoder tools

#17

Fed signal direct from HDHR to Dolby DM100 analyzer and confirms 48 khz no errors.