Longmynd Audio route in Portsdown?

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radiogareth
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Longmynd Audio route in Portsdown?

Post by radiogareth » Mon Apr 13, 2020 8:04 am

I've only really used my MT indoors on the shack PC to watch/listen to traffic on Q100 but yesterday used it successfully to receive DVBS2 using Longmynd on my Portsdown 2018.
I have fitted an internal RTL dongle for LeanDVB/WB FM listening etc when out portable, and I used the RPI audio out jack which works on WBFM RX.

I have searched around but can't find reference to the audio route from MT to Longmynd on a RPI to the outside world. Does it route through the 'selected' RPi output?

TIA

Gareth G4XAT

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Re: Longmynd Audio route in Portsdown?

Post by G8GKQ » Mon Apr 13, 2020 9:48 pm

Hi Gareth

You need to use the "OMX Player" button to get audio out of the LongMynd receiver on Portsdown. You can then select between the RPi audio jack and an external USB dongle using one of the buttons on the Receiver "Config" menu. The default is the RPi audio jack.

Dave, G8GKQ

radiogareth
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Re: Longmynd Audio route in Portsdown?

Post by radiogareth » Tue Apr 14, 2020 7:18 am

Thanks :-)
Gareth

radiogareth
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Re: Longmynd Audio route in Portsdown?

Post by radiogareth » Thu Apr 16, 2020 7:19 am

Now I can hear transmitted audio from my Longmynd, its about time I added audio to my transmit capability. When I built my Portsdown I included an Easycap dongle (uncased so I could stack it up with an (also uncased as they run hot) RTL Dongle.
I've a lot of ex-early car-phone kit Electret mics in nice little plastic cases with 'sun visor' clips on the back.
How should I use an electret with the easycap input, currently wired direct to a front panel phono socket.

I'm thinking add 5 volts via a 10K resistor the the plug centre and a DC blocking capacitor (?100nF) between it and the Easycap input?

Guidance welcome :-)

Thanks

Gareth G4XAT

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Re: Longmynd Audio route in Portsdown?

Post by G8GKQ » Thu Apr 16, 2020 10:58 am

Hi Gareth

Not sure that the audio level from your proposed arrangement would be high enough to drive an EasyCap properly.

I suggest that you purchase one (or 2) of these cheap little USB Audio dongles on eBay. They are designed for the job (with power on the ring and signal on the tip), and the Portsdown is designed for them.

I bought 2 a few weeks ago from a UK seller and they arrived within a few days.

Dave
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G4FRE
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Re: Longmynd Audio route in Portsdown?

Post by G4FRE » Thu Apr 16, 2020 12:04 pm

I experienced a similar issue with getting enough audio to drive the audiocap a few months ago. Then I came across G3AAF selling the old SL6270 vogad chip at the Swindon rally. G4JNT had published a circuit using that chip and an opamp to drive a PWM modulator in Radcom April 2013 page 40. All it needed was a dc blocking capacitor on the outputof the LM358. The only issue with the circuit is that the 4u7 capacitor across the 1M resistor needed to be a 47uF as per the datasheet (andy confirmed it was a radcom transcription error). Using my heil headset I did need the 2k4 resistor due to its high output. It worked so well I got a few PCB made and populated.

I subsequently discovered a 5.6GHz (non atv) use. It is an ideal circuit to generate enough audio to drive the FPV transmitters with enough audio to have decent voice qsos

Dave
G4FRE
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Last edited by G4FRE on Thu Apr 16, 2020 9:36 pm, edited 2 times in total.

G8GKQ
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Re: Longmynd Audio route in Portsdown?

Post by G8GKQ » Thu Apr 16, 2020 3:49 pm

Great idea Dave

What RadCom issue was that in? I have just found an SL6270C in my junkbox - now destined for my Langstone!

Dave, G8GKQ

radiogareth
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Re: Longmynd Audio route in Portsdown?

Post by radiogareth » Thu Apr 16, 2020 8:07 pm

Thanks for the pointers: Had a rummage and found a RPI 'audio' dongle so I will give that a try and see what levels I get.
Had a look inside the mic assembly and found a lovely PCB with what I think is a low power single rail opamp 33171 https://docs.rs-online.com/a748/0900766b815744ea.pdf. The PCB has the motorola logo on the back so I'm assuming its exactly that, an amp, possibly with a bit of AGC in the circuitry. There are only two wires to it though, so I'm wondering how power and audio can go up/down the 'live' wire. Unless its like an electret, where the voltage drop via the feeder resistor IS the audio signal? So a bit like DC up the coax, only at audio. How to do THAT??
Be nice to use as-is. Worst case I suppose would be to change the feed wire to 3 core and add a specific 3.5 stereo jack suitable repurposed. The large electrolytic is 16V rated.
2020-04-16 20.56.06.jpg
2020-04-16 20.56.06.jpg (211.1 KiB) Viewed 430 times
Thoughts??

Gareth

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