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.
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