Raspberry Pi XBMC and DTS Audio
by adama on Jul.07, 2012, under Linux, Overclocking, Raspberry Pi, XBMC
Raspberry Pi
I finally got a Raspberry Pi last week. To my shame I caved in and bought one from Ebay for £57 inc delivery. I think it’s probably still worth that much though. The Raspberry Pi, for anyone who’s been living under a rock, is a credit card-sized ARM-based computer which sells for $35 or about £30 including delivery.
Specifications include:
Raspberry Pi Board
Broadcom BCM2835 SoC
CPU – 700MHz ARM1176JZF ARMv6 (ARM11)
GPU – 250MHz VideoCore IV (OpenGL ES 2.0, 1080p h.264 L4.1 40Mbps, MPEG-4)
SDRAM – 256MB
USB Ethernet controller + 2 USB ports
HDMI + Composite + DSI
SD slot
8xGPIO, UART, I2C, SPI.
All this is powered from a single 700mA (3.5w at 5v) MicroUSB connection.
I’d been planning to try it out as an XBMC media centre and a Linux/Samba file server. The ARM is a little slow but being able to do H.264 and MPEG4 ASP in should hopefully ensure CPU won’t hold it back too much when playing media.
XBMC
XBMC Main Screen
Getting XBMC on the RasPi was simple, just download a Windows (or Mac or Linux) autoinstaller from raspbmc.com (currently down) which will install a bootstrap to your SD card. You then boot up your RasPi with that in the slot and it automatically downloads the latest build of RaspBMC, installs it onto your card and reboots into XBMC.
The first thing I noticed was that the background images have been removed from the default Confluence skin and replaced with a single static image for all options, I assume this is to reduce memory and CPU load on the poor ARM. The interface is usable, but obviously not as quick as my Atom/ION machine.
Initially I attached the RaspBMC to my XBMC MySQL database, but the interface became a little too slow to be usable. I’ll have to try this again later to see if I can speed it up.
XBMC Summary
When I came to try some content nothing would play smoothly at 1080p and only about half of the 720p content I tried would play smoothly. It turns out that the ARM CPU just doesn’t have enough grunt to decode the DTS audio found on almost all 1080p and most 720p content. Almost XviD content I tried played fine, one or two had some hiccuping and one or two just refused to play, I think this is probably related to the encoding features used.
The situation is a little better for content played from a USB-attached hard disk or flash drive, but it’s still not quite fast enough to play the DTS streams. It seems that the USB network adaptor is a bit of a CPU hog.
Apparently this is a common irritation for people wanting to use the RasPi as a media centre. That, and the lack of hardware MPEG2 decoding making it impossible to decode many HD broadcast streams. This isn’t an issue for me, and anyone who really needs MPEG2 decoding will probably have other options.
Overclocking and Raspbian armhf
To fix the DTS issue I decided to try to overclock the Pi. It has a little headroom for overclocking, the creators say that almost all Pis should be able to hit 800MHz and many are reported to go much faster.
See the post on Raspberry Pi Overclocking for more details.
The highest overclock I achieved which was stable in XBMC was 1050MHz on the ARM core, 450MHz on the RAM and 500MHz on the GPU core. At those speeds the Pi still couldn’t quite decode DTS audio streams.
I also installed Raspbian, which uses armhf and provides a significant performance improvement on the RasPi. See the post on Raspbian Benchmarking – armel vs armhf for more details.
Breakdown of audio decoding performance for various configurations of the Raspberry Pi
I tested the performance of the Pi at decoding various audio formats on the armel and armhf architectures and with the stable clock. The chart above shows the %age of CPU required to decode the various audio formats in real time.
Using Raspbian overclocked to 1GHz and my XBMC binaries I was able to play back a relatively high-bitrate H.264 (~20Mbps) video file with 1.5Mbit DTS audio successfully from a USB hard disk and from a local samba file server with relatively high success rates, but still not perfectly. There still seems to be problems when the video bitrate goes above ~20Mbit. Note that the core_freq=500 overclock was important, without it the playback still stuttered.
Below are the config.txt settings for my highest stable XBMC overclock.
arm_freq=1050
core_freq=500
sdram_freq=450
over_voltage=6
over_voltage_sdram=0
disable_overscan=1
Audio Transcoding
I finally decided that I would transcode all of my DTS audio-streams to an alternative format so that they’ll play properly on the RPi over the network. I’m no audiophile and I don’t have a fancy home theater setup, so I decided to transcode all of the audiostreams to 192Kbps stereo AAC, which should surpass 256Kbit MP3 in quality. If you wanted to retain your 5.1 surround you could use 5.1 448Kbps AAC, which the RasPi seems to be able decode without issues.
I modified a shell script used to transcode DTS to AC3. It scans the MKV container and upon detecting a DTS or AC3 file converts them to AAC and re-merges the MKV. I copy the original MKV into a large ramdisk first, as the entire file has to be scanned 3 times during the process.
Transcoding from DTS to AAC can save about 1GB on an average 2 hour file.
Original Filesize: 4,578,516 KB
Extracted DTS Filesize: 1,024,072 KB
Converted AAC Filesize: 114,224 KB
Final Filesize: 3,666,592 KB
Transcoding from AC3 to AAC will save 2-400MB.
Original Filesize: 4,575,228 KB
Extracted AC3 Filesize: 513,364 KB
Converted AAC Filesize: 135,036 KB
Final Filesize: 4,194,832 KB
The transcoding is relatively quick, a 2-3 minutes for AC3 and 5 minutes for DTS.
Of course, none of this would be an issue if you have a DTS/AC3-capable audio decoder, as then the audio would be passed through to the decoder with no processing done on the RasPi.
Conclusion
After an hour’s effort of modifying the script and testing it, I’ve now got a perfectly usable and very low media centre device and as a bonus a shed load of newly-freed up storage space!
I’m quite impressed with the Raspberry Pi so far, even given its shortcomings. I intend to carry one around with me when I’m using hotels so that I can play movies on the hotel TV. I also plan to take a couple to Mexico with me the next time I go over there to act as media centre and file servers in the house.
27 Comments for this entry
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Raspberry Pi Overclocking « memetic dot org
July 10th, 2012 on 00:59[...] adama on Jul.10, 2012, under Overclocling, Raspberry Pi As I posted about in Raspberry Pi XBMC and DTS Audio, the Raspberry Pi is not quite fast enough to decode the DTS audio which often accompanies 1080p [...]
July 13th, 2012 on 22:52
Hi Adama,
Would you mind sharing your script with me?
I’ve got the same problem with a lot of DTS audio Mkv.
Best Regards,
sly
July 15th, 2012 on 16:51
I agree with sly, once you’ve posted a teaser of the transcode script you can’t keep us on a leash like this
July 18th, 2012 on 09:36
Have you tested OpenELEC? It also supports hard floating point operations. Might be faster than raspbmc/raspbian.
http://wiki.openelec.tv/index.php?title=Installing_OpenELEC_on_Raspberry_Pi
July 18th, 2012 on 09:51
I tried darkelec. It seemed to behave oddly, probably because of an old kernel. My installation of XBMC on Raspbian is pretty cut down, I kill off all of the background processes, so there’s nothing else to optimise.
July 27th, 2012 on 23:15
OpenELEC is a totally different implementation of XBMC. Different kernel and OS than Raspbian. There are some reports out there stating that OE performs better, even with DTS fully operational (i.e. http://openelec.tv/forum/124-raspberry-pi/38818-raspberry-pi-feedback#40074). Just think it would be great to have a comparative benchmark ‘OE vs Raspbian+XBMC’.
July 28th, 2012 on 14:19
I find that DTS working or not is a very hit and miss thing. I had it working yesterday, then after pausing the playback it refused to work again! Even a reboot didn’t fix it. I’ve no idea what it is.
There can’t be a great deal of performance difference really. I prefer to not use openelec so i can ssh in and do other things.
July 19th, 2012 on 12:41
Is this possible to have a copy your script ?
July 19th, 2012 on 14:14
at some point i will get around to posting the script. you can find the original by googling, though “mkvdts2ac3″. i merely modified it to convert to AAC (as AC3 is a bit crap)
July 22nd, 2012 on 09:13
niceeeee
July 26th, 2012 on 22:59
Hi,
Could you please explain me why you chose to build your own XBMC instead of rebuilding wheezy’s debian package for armhf ?
I just set up a cross-arch cowbuilder for raspian and I’m currently building “official” XBMC package inside.
For now, it goes well.
Regards, Adam.
July 29th, 2012 on 20:00
Worked great! But I am missing Audio through Hdmi, Hdmi CEC, Airplay and if videos are running I see a piece of the RPi logo at the top and the cursor at the bottom.
Perhaps it is because you used an Android version? :/
July 29th, 2012 on 20:22
Airplay and HDMI CEC are not enabled on this build. HDMI Audio works fine if you follow the guide and don’t fiddle with things unnecessarily (I’ve never had it not work). What kind of stupid suggestion is that? No it’s not the Android version, perhaps it’s because you’re a fool?
September 13th, 2012 on 23:15
Are you planning on updating this package as XBMC makes new beta releases? The iPhone app seems to have stopped working with this “older” version for example?
Thanks for putting it together though, much appreciated.
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January 11th, 2013 on 22:03
alexdombroff@alexanderdombroff.com
February 19th, 2013 on 17:05
Great article. I’ve done the same…jumping through hoops..converting DTS-MA to LPCM to get it playing.
However latest Raspbmc eliminates that step because it will decode DTS stream even at 850Mhz CPU clock.
What I have found problems with is when the DTS-MA track is 96KHz instead of the more common 48KHz. Anyone else experience the same? It still stutters.
February 19th, 2013 on 17:07
BTW…under “Specifications” in your article, doesn’t your board have 512MB of ram not 256MB?
February 19th, 2013 on 18:16
Look at the date of this article and the find the date that the 512MB boards were released…
:>
February 19th, 2013 on 21:50
really?
didn’t know the Model B had a 256MB flavor.
i guess u missed it by a few months.
February 19th, 2013 on 22:16
The Model B was the only one available for ~6 months, and only with 256MB.
Most of the early adopters have those
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