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What happens to The Proms after the Royal Albert Hall?

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Mark Kortekaas Mark Kortekaas | 16:17 UK time, Monday, 19 October 2009

Earlier this year, we broadcast another fantastic season of the BBC Proms. Every concert is broadcast live on BBC Radio 3, with some concerts also broadcast on television - mainly BBC Four, but also BBC HD, BBC Two and BBC One. That meant many live broadcasts live from the Royal Albert Hall - a building which is a number of miles away from Radio 3's studios in Broadcasting House. So how does the audio get from the Royal Albert Hall into my FM/DAB/Internet Radio at home? And what happens to it along the way? How much is the audio in the Royal Albert Hall "dynamically compressed" (where the quiet bits get louder and the louder bits get quieter), and is any of the audio signal chucked away by using bandwidth limiting? And how might you get the best quality from our Proms coverage? I've always been interested in this; so here's what happens: For Radio 3 transmission, on iPlayer and others
  • The Radio 3 stereo mix is sent from the Albert Hall to Broadcasting House via a high-quality 24-bit 48 kHz digital circuit and then fed to Radio 3 FM, DAB, Freeview, Freesat & Online services. The microphones within the Royal Albert Hall handle frequencies from a few Hertz to over 20kHz and there is no LF or HF filtering added to the main microphone feeds.
  • Radio 3 FM is bandwidth-limited to 15 kHz, with DC filtering applied. The FM signal has dynamic range compression applied via an Optimod processor. The signal is NICAM encoded at 676kbps and fed to the FM transmitters via the BBC's distribution network. No further bandwidth limiting is applied.
  • The feed from the Royal Albert Hall is also fed to Radio 3 on DAB, Freeview and Satellite. These operate at 192kbps, although this reduces to 160kbps on DAB at some points in the schedule to accommodate 5 Live Sports Extra on the DAB multiplex. There is no other processing applied to the signal.
  • On the BBC iPlayer's listen-again services, Radio 3 is available at 192kbps AAC. This is processed in the same way as DTT ("Freeview"). Live streaming is also available, at 192kbps Windows Media and other versions.
For BBC Four transmission
  • BBC Four uses the same stereo mix that's used for Radio 3. It's combined with the pictures and sent back to Television Centre via an MPEG2 (MPEG1 Layer II) link at 384kbps. No additional processing is carried out before encoding.
  • BBC Four sound on Freeview, Freesat and Sky is transmitted (using MPEG2) at 256kbps with no processing or bandwidth limiting.
For BBC Two and BBC HD (also BBC One) transmission
  • Proms on BBC Two (and BBC One) use a dedicated sound mixing truck, to ensure that audio is mixed in a complementary way to the pictures broadcast. Proms also transmitted on the BBC HD Channel are usually mixed in surround sound using Dolby 5.0, though broadcast in Dolby 5.1 for technical reasons.
  • Stereo for BBC One, BBC Two and BBC HD is sent back to Television Centre via an MPEG2 (MPEG1 Layer II) link at 384Kbps.
  • When available surround sound is sent back in the same link using Dolby E encoding at 2Mbps (Dolby E can support up to 8 channels).
  • Stereo is transmitted on BBC One, BBC Two at 256kbps (MPEG2), and 256kbps (MPEG4) on BBC HD.
  • Surround sound is transmitted on BBC HD at 384kbps using Dolby Digital encoding. Dolby Digital has a frequency range from about 3 Hz to 18 kHz.
  • Only the surround sound mix is transmitted on BBC HD. If an HD set top box is set to "Stereo" it uses the additional data (Dolby Metadata) we send in the Dolby Digital signal to create a stereo mix.
  • BBC television analogue services use NICAM-728 encoding the stereo signal at 728kbps for transmission.
Of course it should be noted that various transmission chains have their own issues depending on the output. For instance the Freeview signal is MPEG coded, filtering happens as part of the coding process, in a 32 segment polyphase band pass filter. The AAC encoding does its own thing, etc. With that in mind, the feeds are really filtered to you as the end listener. I'm grateful to Andy Quested from BBC HD, and Neil Pemberton from BBC Radio 3 for compiling these answers.

Comments

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  • 1. At 10:25am on 20 Oct 2009, HD1080 wrote:

    Why aren't things like the Last Night of The Proms released on Blu-ray too?

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  • 2. At 10:30am on 20 Oct 2009, HD1080 wrote:

    Also, are they going to ever replace the opera singers on it with normal singers instead please?

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  • 3. At 10:35am on 20 Oct 2009, rhughes2832 wrote:

    Very interesting - thank you!
    One thing I'm still wondering - what is the format of the audio in the BBC HD version on iPlayer? Is this the 5 channel version?
    Which would be the highest quality on iPlayer, the audio stream of BBC HD, or the 192k AAC stream from the Radio 3 broadcast?

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  • 4. At 11:48am on 20 Oct 2009, smiler_jerg wrote:

    I expected a rather different article from the title, but this was very interesting. As far as radio broadcasts go, I pretty much knew it already, though the DAB bandwidths were interesting regarding R5LE. I'd be interested to know just how much dynamic range compression is applied to FM broadcasts. It can't be much; unfortunately my reception was pretty poor this summer, and quiet passages were inaudible thanks to interference! I once read that Radio 3 has the greatest dynamic range of any FM station in the UK.

    What I was expecting from the title: details of what happens to the recorded footage when the Proms season is over. Think of all those unique concerts — the premieres, special commissions, one-off performances — that have occurred over the recorded history of the Proms. How much of this has been kept? I look forward to the day when any Proms performance from the last 50-70 years could be called up on iPlayer.

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  • 5. At 4:29pm on 20 Oct 2009, alanogilvie wrote:

    Web iPlayer - 'HD' content - audio is presented as 192kbps stereo AAC-LC.

    iPlayer 'HD' on Virgin Media is presented as 256kbps MPEG1 Layer 2 audio.

    So - in answer to @rhughes2832 - the Web iPlayer 'HD' is equivalent of the R3 Simulcast stream.

    Interesting comparison between 192kbps AAC-LC and 256kbps MP1L2 - in prior listening test we've done (so not a specific test for this comparison) I would find an 'average' ear not hearing much difference between those two. Although a 'golden' ear would hear certain compression artefacts.

    Alan

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  • 6. At 4:01pm on 22 Oct 2009, Kampernaut wrote:

    Mark/Alan,

    Please would you be kind enough to explain why you use AAC inside flash on iPlayer and WMA for live radio internet streams?

    I've noticed that the Five Live WMA stream is usually significantly behind every other way of listening including iPlayer, Sky, DAB and AM. It's not just a little bit either (30-60 seconds).

    Are you planning to make the AAC streams directly accessible?

    Many thanks

    K

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  • 7. At 11:58am on 30 Oct 2009, a new user wrote:

    LOL

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  • 8. At 11:59am on 30 Oct 2009, mpg plain wrote:

    LOL-2

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  • 9. At 6:32pm on 02 Nov 2009, mak51 wrote:

    You say : "On the BBC iPlayer's listen-again services, Radio 3 is available at 192kbps AAC".
    On the iPlayer messageboards a couple of weeks ago I asked why the pop-up on R3 listen-again streams shows 128kbps - as eg.:
    "BBC Media Player v.2.20.14200.14320
    b00n6zbx | 128kbps | aac | LI"
    Though you responded briefly, asking for more details, you haven't reported back.. so I'd still be interested to know whether the streams are actually being delivered at 128 or (as another poster suggested) at 192 but simply showing 128 in error.

    Regards

    Walter

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