Author Topic: Headphone Output De-Emphasis  (Read 3892 times)

Storm905

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Headphone Output De-Emphasis
« on: August 24, 2012, 06:13:03 am »
Ii there 50/75us de-emphasis applied to the headphone output, and where do you set it?  Does not sound like it.  If not, can this be added? Cheers

Jan

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Re: Headphone Output De-Emphasis
« Reply #1 on: August 24, 2012, 08:58:10 am »
The audio output does not work as a reference output. Its purpose is different, thus the trebles are emphasized different way that corresponds to 50/75 us deemphasis. In the FM Scope the demodulated signal is showed without deemphasis applied. Deemphasis options will be however added in future version.

For the FM deviation measurements the preemphasis time constant is fully irrelevant as FM deviation limit is defined for a modulation signal that already includes preemphasis. This results into an unorthodox but valid conclusion that from this view preemphasis has only the same role like setting trebles on studio equalizer (that by the way differs station to station).

Nowadays modulation processors with multiple audio dynamics compression stages, multiband processing, lot of settings and other features unfortunately make hard to verify what time constant is really applied in the broadcast chain. More often the most decisive parameter is how does it sound on an average receiver.

Storm905

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Re: Headphone Output De-Emphasis
« Reply #2 on: August 24, 2012, 12:32:59 pm »
Hi Jan.  While the headphone output is not a reference output, it is the none the less useful if it sounds broadly like a average receiver in the local market (i.e. with 50us de-emphasis in this market).   As it can be the source audio device for the pc MP3 recorder, listening to the MP3 files remotely could give a quick and rough indication of possible incorrect pre-emphasis settings in the audio processor, link or transmitter.

I once went to an isolated FM site where another broadcaster had applied pre-emphasis in their processor (with no de-emphasis on output) and then applied pre-emphasis again in their transmitter.  Ran like that for years apparently.  Sounded shocking and of course with relatively low modulation power to accommodate for the 2nd pre-emphasis boost.  A great case for a P175 left on site and a better set of ears to listen to the MP3 files remotely...

Jan

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Re: Headphone Output De-Emphasis
« Reply #3 on: August 27, 2012, 12:48:44 pm »
See the frequency characteristic attached. Similar to spectrum analyzers the audio output has some specific purpose. The P175 effectively amplifies trebles by about 5 dB as the original idea was to make noise more audible. Note that the device is not an FM tuner but it is a measuring equipment, thus its internal structure differs from any FM receiver. Adding high quality flat stereo audio output would require implementing parallel FM receiver and exceeding the budget that now allows to offer this device below 200 EUR.
Your need to recognize double pre-emphasis or no pre-emphasis is not affected. As noted before, an absolute value of attenuation is not so relevant. More important is the relative difference and ability to recognize it, that is unaffected of course. Pre-emphasis failure is something you usually cannot directly measure "from air" but you can recognize by ears if you use the same receiver for a long time.

Storm905

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Re: Headphone Output De-Emphasis
« Reply #4 on: August 28, 2012, 02:36:12 pm »
That make sense, thanks.  I understand high quality stereo output is not possible or intended in the price range of the unit.   

For those using the FM Scope/Guard MP3 recorder (or a real time MP3 streamer for distant P175's), the P175 will typically be the audio source for that, and while the audio does not have to be stereo, nor be accurately flat/de-emphasised,  it is desirable (if its easy i.e. without extra hardware, cost, etc), that the audio spectral balance of the MP3 recordings retrieved roughly sound like an average or budget FM tuner.  Remote monitoring of the MP3 audio may not always be done by someone familiar with the treble boost applied in the P175. 

Sure, if the ~5db P175 treble boost is hardware fixed, then ok that's understood.  If it's applied via DSP then a rough ~50/75 de-emphasis curve could still be useful  **if easily done in software** at a future time.

Not important though.  For now its useful knowing there is a boost, and the idea behind it. Thanks.