Do they think we do not hear the difference?

[Gordon]

Thank you Gordon for this explanation about clapping, it makes a lot of sense. Does this mean that the Nicam distribution chain to the FM transmitters can cope because it is a much less lossy system?

Yes! NICAM is inherently PCM [14 bits to 10 using a sliding scale scheme] with no loss so it will give a good rendition of noise and add relatively little of its own quantising noise which is very low, unlike that of compressed audio [that makes a lot of noise but tries to redistribute it in places in the spectrum where ears can't hear it very well if at all].

The NICAM system is getting a bit old now so perhaps, as a part of the build up to DSO, the BBC would consider using the DAB signals to feed the FM transmitters to save money. Why not put a PURE portable in each transmitter and then feed the FM modulator from that!!!!

RM: A version of NICAM is still used to feed the FM transmitters.

[Nick_G]

Yes! NICAM is inherently PCM [14 bits to 10 using a sliding scale scheme] with no loss so it will give a good rendition of noise and add relatively little of its own quantising noise which is very low, unlike that of compressed audio [that makes a lot of noise but tries to redistribute it in places in the spectrum where ears can't hear it very well if at all].

The NICAM system is getting a bit old now so perhaps, as a part of the build up to DSO, the BBC would consider using the DAB signals to feed the FM transmitters to save money. Why not put a PURE portable in each transmitter and then feed the FM modulator from that!!!!

RM: A version of NICAM is still used to feed the FM transmitters.

Don't say that Gordon. Some people seem to think that the BBC already do this but thankfully they are misinformed. But the thought of this happening is almost too hideous to contemplate

Regards,
Nick

[Gordon]

Don't say that Gordon.

For the avoidance of doubt I was joking, but it is possible.

BTW another random thought, if they did they'd probably need to keep Optimod because of DAB's dynamic range!! What about that: DAB + PLUS Optimod!!

[NHTL]

The NICAM system is getting a bit old now so perhaps, as a part of the build up to DSO, the BBC would consider using the DAB signals to feed the FM transmitters to save money. Why not put a PURE portable in each transmitter and then feed the FM modulator from that!!!!

RM: A version of NICAM is still used to feed the FM transmitters.

I like it, but we must not tempt fait!

Thank you for clarifying the Nicam feeds to transmitters. If you look at this site you will see some photographs that I took of the Nicam feeds, amongst other things, at the North Hessary Tor transmitter when Radio 1 started to broadcast in FM from that transmitter. I was friendly with the engineers of the time and they invited me to the opening. My interest was always with Radio 3 I hasten to add!

http://tx.mb21.co.uk/gallery/gallerypage.php?txid=154&pageid=1131

[Gordon]

I like it, but we must not tempt fait!

Thank you for clarifying the Nicam feeds to transmitters. If you look at this site you will see some photographs that I took of the Nicam feeds, amongst other things, at the North Hessary Tor transmitter when Radio 1 started to broadcast in FM from that transmitter. I was friendly with the engineers of the time and they invited me to the opening. My interest was always with Radio 3 I hasten to add!

http://tx.mb21.co.uk/gallery/gallerypage.php?txid=154&pageid=1131

Wow!! great stuff NHTL. Believe it or not in the second photo those big black dustbins are filters, part of the plumbing feeds to the aerials up aloft!!

I dont suppose that your pal knew Keith Hayler or Graham Sawdy did he?

[NHTL]

I dont suppose that your pal knew Keith Hayler or Graham Sawdy did he?

I do not know Gordon, but I will try to find out.

[rank_and_file]

Some very impressive technical posts, going way above my head, but I do have a couple of observations.

Are we talking about fairly recent experiences, say over the last 5 years, of the FM bandwidth sound quality? My understanding is that the BBC version of Optimod has been on 24 hours a day over that period or longer. Is this true?

In other words, about 15 years ago when I added a NAT01 tuner to my Naim system, the Optimod system was not in use, or only in use for the “drive” programmes. (I remember vividly Kenyon promising that Optimod would ONLY be used for those programmes due to cars/tyre noise). I remember being pretty impressed with the pre-Optimod FM sound in those earlier days: nowadays it is very ordinary.

Picking up on what Bryn said earlier in the thread, listening to iPayer HD produces a far superior sound to FM under OptiMod - listened to “Parsifal” last afternoon/night - excellent sound all the way from New York! So if the HD streaming is going to be fairly equivalent to DAB+ I am all for it. However, as others say, I think, DAB is not able to be upgraded to DAB+, or has this problem now been overcome?

What chance, I wonder, of the BBC rolling out DAB +, and not DAB?

[Gordon]

Some very impressive technical posts, going way above my head, but I do have a couple of observations.

Are we talking about fairly recent experiences, say over the last 5 years, of the FM bandwidth sound quality? My understanding is that the BBC version of Optimod has been on 24 hours a day over that period or longer. Is this true?

In other words, about 15 years ago when I added a NAT01 tuner to my Naim system, the Optimod system was not in use, or only in use for the “drive” programmes. (I remember vividly Kenyon promising that Optimod would ONLY be used for those programmes due to cars/tyre noise). I remember being pretty impressed with the pre-Optimod FM sound in those earlier days: nowadays it is very ordinary.

Fundamentally FM has not changed since the 1960s, apart from OptiMOd and refurbishment of transmitters and distribution with NICAM in the 1970s and then RDS. I don't remember exacly when they started using OptiMod perhaps Bryn can? Before it someone had to keep an eye on a meter and some protective means at the transmitters. I'm sure it was more than 5 years and, as you say, at first when complaints came in it was promised that it would be applied lightly to R3. It's settings do change over the day I think. OptiMod does have some good features that protect FM it's just that excessive use of it is audible to many. As you say, there's the issue of in car listenng too - excessive dynamic range gets complaints too from the other end of the listening scale!! You can't win.

Picking up on what Bryn said earlier in the thread, listening to iPayer HD produces a far superior sound to FM under OptiMod - listened to “Parsifal” last afternoon/night - excellent sound all the way from New York! So if the HD streaming is going to be fairly equivalent to DAB+ I am all for it. However, as others say, I think, DAB is not able to be upgraded to DAB+, or has this problem now been overcome?

The transmission system used for DAB is also suitable for DAB+ and the two can be mixed at will in a given multiplex. The problem is not fundamentally technical. It is in getting the broadcasters fired up with the idea and also having a transition strategy supported by goverment and OfCom. The conditions for DSO include one that a receiver sold carrying the switchover Tick mark [like for TV DSO] must be able to support DAB+. Some are already in the market with DAB+ readiness. So from the announcement of switchover there will a growing number of receivers capable of DAB+ = but no transmissions!!

The DAB+ used in the HD feed is at 320 kBits/s I doubt that BBC DAB multiplex capacity [max 2 MBits/s gross] will afford that much.

What chance, I wonder, of the BBC rolling out DAB +, and not DAB?

Very small at this moment there seems no enthusiasm for it among broadasters, although there is technical interst in the BBC. We don't need to abandon DAB as such BUT of course there is a large body of DAB only receivers out there that cannot get DAB+ and cannot be dumped and so the problem will be spectrum to allow some simulcasting [= yet more transmission costs] so that DAB can be phased out over a period. It needs a long term strategy so with current governments don't hold your breath.

[MrGongGong]

There is a very interesting parallel discussion going on on the CEC list (Canadian Electroacoustic Community) about what the basic quality thresholds are these days. Most folk arguing for 24 bit / 48kHz as a baseline.

[Resurrection Man]

Nice photos, NHTL. Mind you, I always did think while working at the BBC that management were a bit faceless. Now I know I was right!

[Nick_G]

For the avoidance of doubt I was joking, but it is possible.

BTW another random thought, if they did they'd probably need to keep Optimod because of DAB's dynamic range!! What about that: DAB + PLUS Optimod!!

Oh I realise you were joking Gordon but I could almost see one of the bean-counters suggesting it to save money and to persuade people to move away from using FM...

Regards,
Nick

[Gordon]

There is a very interesting parallel discussion going on on the CEC list (Canadian Electroacoustic Community) about what the basic quality thresholds are these days. Most folk arguing for 24 bit / 48kHz as a baseline.

And why not!! I think that the 48 is more desirable than the 24. Even with this sampling frequency the audio bandwidth is only 24kHz so the phase and filtering issues discussed above are not necessarily made much easier. The AES 24 bit quantisation is meant to deal with production issues and is arguably too much for domestic purposes but I'm sure that there are those that would disgree with that. The more bits and sampling frequency the greater the transmission capacity needed. To preserve that PCM 48/24 bandwidth in transmission one would need to have proportionally higher bit rates; even in compressed form state of the art algorithms have more work to do to deliver performance that will not excessively degrade the source 48/24, hence BBC streaming going to 320.

[rank_and_file]

Gordon

Many thanks for your interesting information.

A slight tangent on DAB+. Is it backwardly compatible with DAB radios?

Was not one of the sticking points on the roll out of DAB that, a couple of years ago, no cars had them installed? This is now happening, and perhaps listeners with very modern cars have a DAB, or a DAB+ radio fitted. I understand that the rest of Europe went DAB+ from the start so, am presuming, European car manufacturers might be fitting DAB+ radios?

[Gordon]

Gordon

Many thanks for your interesting information.

A slight tangent on DAB+. Is it backwardly compatible with DAB radios?

Was not one of the sticking points on the roll out of DAB that, a couple of years ago, no cars had them installed? This is now happening, and perhaps listeners with very modern cars have a DAB, or a DAB+ radio fitted. I understand that the rest of Europe went DAB+ from the start so, am presuming, European car manufacturers might be fitting DAB+ radios?

DAB radios are not usually back compatible. As I said there are some out there that have DAB+ already installed but you'd have to look for them specifically. Sony I think have most if not all theirs DAB+ ready. Older sets almost certainly won't be.

New cars will from this year all be FM/DAB/DAB+ ready [according to the trade association SMMT] but there's all those old ones that aren't. This is partly the case because there is a European ruling about it and in other parts of Europe DAB+ has been in sets from the start.

[Gordon]

Sticking to low-pass filters, in any single order filter the frequency response will start to change at the design frequency, be 3 dB down at that point, and be asymptotic to a 6 dB/octave response change. At the rollover point the phase change will be 45 degrees and eventually attain 90 degrees. Thus, about the rollover frequency there's varying phase changes being applied to the audio signal: phase distortion thus occurs at least as far down as one octave below the rollover frequency. In the case of LP equalisation I stated that there's a constant phase change applied across almost the entire audio frequency spectrum by the equalisation filter. In the frequency domain there's a 6 db/octave roll off; in the phase domain there's a constant 90 degree shift. On that basis there won't be any phase distortion; square waves will be well handled by LP.

Sorry, I must have missed this post in all that obsession with FM tuners!! At the risk of making this thread even more nerdy!! :

Yes, I See. The RIAA shaping is made by a very simple single zero/pole/zero network so the phase shifts are very small anyway [compared to sharp cut filters]. The shaping at the cutting machine is probably minimum phase at this stage but will have some heavy filtering beyond the audio band to protect the cutter amplifiers etc and these may be fast cut off and, as you say, will have phase shift effects into the audio band. These filters are not naturally phase linear but could be made so very easily but at a cost.

It doesn’t matter because, provided the channel from cutter via the LP playback to the pre amp where the inverse RIAA is located, is clear of severe amplitude and phase errors etc, those RIAA phase shifts will be reversed and bring the signal back to where it was when entering the cutter. The cartridge and cutter and the vinyl itself then come into question and who knows what their phase responses are!! If it was important and we knew what they were we could correct them. FM has this issue too – see below.

Given that in the analogue era the cutter was fed from a tape machine [itself fed from a mixing desk and that by a microphone system] which used pre emphasis and other head response correction circuits, goodness knows what its phase response would have been. Furthermore, master tapes could well have passed via copies. And then there is the multi-unit loudspeaker phase response [crossover networks??] as the sound gets to the ear…….it never ends.

As for LP managing square waves well there are other things going on that would make that tricky but for the electronic part of the path that is probably true provided the “square” wave is not ideal but has rise times compatible with the channel bandwidth. We are back to transient issues.

In the case of CD there's a filter created by the 44 kHz sampling rate which will apply an abrupt frequency cut at 22 kHz. This will have a roll off rate far steeper than a single order filter and of course make for a phase change at frequencies above 22 kHz and is bound to create phase changes below as well. But because the CD filter is so steep, many octaves below the 22 kHz roll-off there'll be significant and varying phase changes. Can this be corrected by applying many high order filters of opposite effect? As for correction by over sampling, is there any proof this will correct the phase distortion? Hence, across the frequency spectrum CD is unlikely to be phase coherent. Transients will not be handled very well by CD.

Yes,-ish. The coding and decoding filters should ideally be linear phase so that there is constant delay. This is perfectly practical even for fast roll offs [although it takes additional components] and any decent coding systems and CD players should do that. However the fact that the bandwidth is sharply defined is the basic issue.

If one over-samples at the coding end then the need for steep roll offs is reduced at that point. BUT anti-aliasing is still necessary but now the filters need not be so severe and ringing contained. Over sampling supports a wider base bandwidth, out as far as you like with an appropriate sampling frequency.

Once in the digital domain arithmetic processes take over the implementation of filtering for CD mastering to 44.1/16. In the initial over–sampling one must be sure to quantise with more than 16 bits too so that arithmetical computation round off errors can be absorbed and noise shaping done. [Think of what is happening in a digital mixer desk if there is one dominant mic feed taking say 18 bits of AES with 6 as headroom. What is happening to another mic that is set say -6dB relative to that.] The benefits of over sampling and >16 bits are obvious in audio acquisition and post-production editing etc. In CD playback they are more limited.

There is no magical solution to this filtering given that 44.1/16 is the ultimate target for general distribution on CD. Regardless which way it is done those constraints still apply and so the problem now moves to the design of the down-sampling filters which have to maintain the flattest, widest possible bandwidth whilst not bumping into aliasing at 22kHz. Now it MAY be easier to design practical, phase corrected FIR filters in the digital domain than to make them in the analogue but they will still ring if they are too fast in roll off. Same argument applies to the player which has only the 44.1/16 format to work with. The only reason that over sampling is useful at the player is in implementing that Nyquist reconstruction filter, it will not necessarily improve the ringing which has been embedded in the bits. Removing the ringing implies removing from the audio that energy which causes the ringing, ie soften the audio by starting a playback filter rolling off at about 10 kHz, very softly. An alternative is to add back the missing frequency components that were removed in the down-sampling. Then again a touch of harmonic distortion would get part way there!! Perhaps that's why FM sounds better than it should!!

Now in the AAC+ standard [endorsed by auiophiles] there is a technique that attempts to do this – SBR. The audio is filtered to half its bandwidth then compressed and the decoder invents and inserts an approximation to that missing top half!! It sounds a dreadful thing to do but people like it. If the CD playback electronics were given access to knowledge of the down-sampling filtering then SBR might improve CD!

http://telos-systems.com/techtalk/aacplus/aacPlus_overview.pdf

http://en.wikipedia.org/wiki/Spectral_band_replication

So I would agree that CD will have transient issues attributable only to the band limiting. But then, as I have been saying, so does FM which also has to restrict band widths before modulation. Given it also has other issues relating to amplitude control, it isn’t that good at loud transients either [enter OptiMod]. An FM modulator also has a pre-emphasis network consisting of a single zero that has a complementary pole at the receiver so p/e will not have inherent phase issues, again provided there are no issues in the channel in between them, like a high power [250kW] transmitter amplifier.