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Old 5th November 2014, 07:59 PM   #1
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Default 'Warming up' digital radio

I've listened to a number of different DAB tuners connected to a number of different DACs, but haven't been able to achieve a 'warm & relaxing' sound that works for extended listening. No doubt the low bitrate MP2 encoding is to blame. My Linn FM tuner is a joy in comparison. 95% of my listening is BBC Radio 4, which is pretty much voice only.

I'd like to build an audio 'stage' specifically designed to add warmth to MP2 voice radio. I've thought about the following, but would appreciate advice on which would have the best chance of success:

- EQ (roll off top end, fill out kids?)
- old steel (poor spec) audio transformer
- tube stage with lots of H2

Please help me with my deliberately lo-fi project
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Old 5th November 2014, 08:31 PM   #2
Julf is offline Julf  Europe
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Quote:
Originally Posted by sharpi31 View Post
I've listened to a number of different DAB tuners connected to a number of different DACs, but haven't been able to achieve a 'warm & relaxing' sound that works for extended listening. No doubt the low bitrate MP2 encoding is to blame. My Linn FM tuner is a joy in comparison.
You do realize the FM transmission is based on a fairly low-bandwidth digital encoding too?

Quote:
I'd like to build an audio 'stage' specifically designed to add warmth to MP2 voice radio. I've thought about the following, but would appreciate advice on which would have the best chance of success:

- EQ (roll off top end, fill out kids?)
- old steel (poor spec) audio transformer
- tube stage with lots of H2

Please help me with my deliberately lo-fi project
Yes, either an old tube amp, or a digital DSP plug-in would do it. I would go the DSP path, using a raspberry pi or one of the newer small, cheap embedded linux units.
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Old 5th November 2014, 08:40 PM   #3
JMFahey is online now JMFahey  Argentina
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Go straight to the point.
If you are interested in voice only (by the way, I also listen a lot to voice BBC podcasts , also to Voice radio from all over the World, often with very low quality, through Shoutcast) and find low bitrate digital distortion annoying, you can add , say, a 12dB/oct or better 5KHz lowpass before the last power amp.
It won't affect intelligibility at all and will definitely sound smoother.
And you can easily do it with a couple Op Amps.
Search Op Amp datasheets which always include some or google active lowpass filter.
Post results.

No need to go for intrinsic rolloff of poor audio stages, transformers, etc. when you can custom add your own ... and switch it off the way if you listen to a better program material.
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Old 6th November 2014, 01:09 PM   #4
DF96 is offline DF96  England
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I seem to recall reading somewhere that the typical UK DAB station has little or no program content above about 11kHz, but may still have encoder artifacts around and above there. If so, a low pass filter around 10kHz may help. You will never achieve a truly 'warm and relaxing sound' using DAB, as the bit rates now used have been found to result in subjectively 'annoying' sound. The best you can do is reduce the annoyance.

It is true that FM audio is sent via a digital distribution chain but this is much better than low rate MP2. I believe the BBC still uses its Nicam system for distribution. This is a 13-bit non-linear system (using 32kHz sampling rate?) but there is no perceptual coding; many years experience shows that it can provide high quality audio. The limited bandwidth would be required by the Zenith-GE stereo system anyway, so the main limit on FM fidelity is imposed by the transmitter modulator linearity(which we can assume to be good), the receiver discriminator linearity (?), the IF bandwidth and its smoothness (?), and multipath reception.
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Old 6th November 2014, 01:25 PM   #5
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Thanks all for the info. I was aware of the digital encoding upstream of FM broadcast but understood that this was less 'damaging' than DAB processing (it certainly sounds 'less bad' to my ears).

I have a DEQ2496 that I can insert between DAB receiver and DAC, so it will be easy to test the recommended low pass filters. It would be nice if a 12dB or steeper low pass in the 5-15K range made a significant improvement. If it worked, I'd be tempted to implement this in the analog domain on one preamp input.

The speakers I'm currently using cross over to B&G Neo3 planar tweeters at 2KHz so the low pass filter effects should be easy to hear.

Will report back
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