De-emphasis 75 µs or 50 µs for FM

[DB9JG]

There seems to be a default 75µs de-emphasis setting, which addresses the FM emphasis used by U.S. FM stations. However, the rest of the world uses a 50µs FM emphasis setting (AN338 page 39).

Many MiniRadio users complain about missing bass audio frequencies and assume the low values for the coupling capacitors are the cause. Could this be an effect of the receiver not being set to a 50µs de-emphasis while receiving non-U.S. FM stations?

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Question: What happens to the sound quality if an FM receiver is set to 75 µs de-emphasis but used in Europe where 50 µs de-emphasis is standard?

Short answer: The sound will be too bright and harsh because the receiver's de-emphasis does not match the transmitted 50 µs pre-emphasis standard in Europe, causing distortion of the audio balance and overall tonal quality.
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From AN338:
Property 0x1100. FM_DEEMPHASIS
Sets the FM Receiver de-emphasis to 50 or 75 µs. The CTS bit (and optional interrupt) is set when it is safe to send
the next command. This property may only be set or read when in powerup mode. The default is 75 µs.

Could this be an issue with the current firmwares?

[max-arnold]

Hi Peter,

It is supported since v2.23 (Settings->FM Region) and defaults to 50µs, see #116 for more details.

[DB9JG]

Thanks Max,

I didn’t find that with my first search:-(

I came about that topic when looking at the AM settings where a 100Hz high pass is mentioned:

[G8PTN]

Hi Peter,

On the original V1 hardware without the headphone amplifier, even when driving an AUX input compatible powered speaker with a nominal 2kΩ impedance, with the 100nF coupling capacitors, fcutoff is approximately 800Hz, so there will be little or no bass. If headphones are connected directly, as well as the level being lower, the cutoff frequency will be even higher.

I assume the original V1 design which uses the NS4160 amplifier to drive the speaker, based the capacitor values on the example from the datasheet, and did not consider the headphone output connection.

With the hardware incorporating the headphone amplifier, the 100nF coupling capacitors from the SI4732-A10 audio outputs would probably be acceptable.

73, Dave

[DB9JG]

Thanks Dave,

Yes, the 10K impedance of the reference design makes it respond much better at lower frequencies.

Let's talk about the impedances at much higher frequencies: I feel we’re all waiting for the V4 to be reverse engineered ( -again no schematic circuit available-) and are eagerly anticipating new issues from the headphone wire antenna and its influence on the shortwave's path.

It’s like going two steps forward and one back again :-)

[G8PTN]

It’s like going two steps forward and one back again :-)

Hi Peter,
Yes, although there have been some minor incremental improvements, I suspect the modifications will not have been thoroughly tested. It’s a neat marketing method, since any change will generate additional interest and further sales in a never ending cycle.

73, Dave

[DB9JG]

That's exactly what I think about the changes and the reason for the changes.

The donut antennas are the best example:
“10kHz - 180MHz” was very promising, but it was topped by
“9.9kHz - 181MHz”.

My marketing idea now was
“9.8kHz - 182MHz… even suitable as a lightning rod and a somewhat limited fly swatter”.

Well ... I'll miss the theremin effect :-) and I am happy with the protection diodes.