• Effective immediately we will be deleting, without notice, any negative threads or posts that deal with the use of encryption and streaming of scanner audio.

    We've noticed a huge increase in rants and negative posts that revolve around agencies going to encryption due to the broadcasting of scanner audio on the internet. It's now worn out and continues to be the same recycled rants. These rants hijack the threads and derail the conversation. They no longer have a place anywhere on this forum other than in the designated threads in the Rants forum in the Tavern.

    If you violate these guidelines your post will be deleted without notice and an infraction will be issued. We are not against discussion of this issue. You just need to do it in the right place. For example:
    https://forums.radioreference.com/rants/224104-official-thread-live-audio-feeds-scanners-wait-encryption.html

Burst transmitter?

Status
Not open for further replies.

poltergeisty

Deep Thinker
Joined
May 7, 2004
Messages
3,808
Location
RLG, Fly heading 053, intercept 315 DVV
I want to build a burst transmitter for a radio, but haven't got the slightest clue as to how to do it. What would happen is that the audio would be encoded and recorded and sped up to a 1 second pulse and the receiver would slow down the audio back to it's normal length by way of knowing how long the audio was that is encoded.

Right now I use a kinda of poor man's FHSS where the frequency changes among all 22 frequencies on the FRS/GMRS band. Every time you key up a new channel is selected. :lol:
 

slicerwizard

Member
Joined
Sep 19, 2002
Messages
5,340
Location
Toronto, Ontario
Well, let's see - you want to "speed up" your audio and send it in a one second burst...

So you're looking to encode/record about five seconds of audio, then send it in one second? And presumably this procedure repeats as long as you press the PTT button.

So the user on the receiving end doesn't hear your audio until five seconds after you've spoken it. Sending a signal to the moon and back only takes 2.6 seconds. And is annoying as hell. And you think your idea is a good one? And you wonder why you got a strange response...
 

poltergeisty

Deep Thinker
Joined
May 7, 2004
Messages
3,808
Location
RLG, Fly heading 053, intercept 315 DVV
Last edited:

K7MEM

Member
Joined
Dec 16, 2013
Messages
341
Location
1158 W. Valley Circle, Ash Fork, AZ 86320-482
It sounds like a fun idea to play with and think about, but there are a lot of things that you need to take into account. Start thinking more about the required bandwidth.

If your using those links to support the assertion that "the spy agencies use the damn thing", don't. At least, not in the context that your intending. All of those "spy" devices didn't deal with audio. They all talk about Morse Code or RTTY (FSK). Voice doesn't have the power density that these modes have and wouldn't be very useful for critical messages. Their meaning of "high speed" is quite different than yours. Their intent was to transmit a longer more detailed message, at high speed, so that their message could be completed before anyone could locate them. But their pre/post encoding operations were quite lengthy.

If you now take this to the audio range, you need to think about the pre/post steps that need to be done. A simplified approach would be to select a audio bandwidth that maximize intelligibility yet minimize bandwidth, say 1.9 KHz. Then, sample the audio at a rate such that the audio can be put back together later, without losing intelligibility. To then put this on a carrier, you would need to serialize it, so each audio sample will take 8 to 10 times longer to send. If you add error correction, even longer. On the other end, you will have to put this back together and convert back to analog.

You could use compression to squeeze the audio samples into a smaller package, but you would also need to decompress on the other end. Something simple, like a DPCM, could cut your data requirements immediately in half. With DPCM you only send a 4 bit number that represents the encoded difference from one sample to the next. Old technology but still useful.

But by the time your done figuring out the bandwidth required to store 120 minutes of audio, convert it and prep it for a 1 second transmission, you will need in excess of 100 MHz of bandwidth. That easily exceeds the capabilities of any, easily available, RX/TX.

Just as an example of bandwidth requirements, about 35 years ago I worked on a program where we data-linked to an jet flying at 35,000 Ft that was sending us real time radar data. There were two channels of analog (I and Q) information, plus what ever was necessary to track the signal. The bandwidth required for these signals was in excess of 200 MHz. However, we were running at 18 GHZ, so we had the room.

On the receiving end, each received channel had to be sampled by A/D converter that ran with a 100 MHz clock, and the samples then had to be stored on magnetic tape. To do that, the data samples had to be slowed down by a 6:1 de-ramp buffer and then stacked 4 words high (6-bit words) for writing to a 24 channel tape unit. Even at that reduced rate the tape had to run in excess of 150 IPS. Each tape could hold about 15 minutes of data. On playback, if you then slowed the tape down so that the data rate was slow enough for a computer to process the data (1-7/8 IPS) the tape took almost 24 hours to play back.

So I am not saying, don't continue with your idea. All I am saying is to consider the requirements of each device as you work through your idea. Possibly start with greatly reduced requirements and work your way up. And then expect the possibly that the whole idea might just be impractical.
 

K7MEM

Member
Joined
Dec 16, 2013
Messages
341
Location
1158 W. Valley Circle, Ash Fork, AZ 86320-482
I don't understand the bandwidth thing. Basically what I was thinking is that the audio would be sped up to a 1 second burst and then decompressed. What if you had burst audio on FHSS?
Bandwidth is the idea that a radio signal that carries intelligence takes up spectrum space. The amount of space that it takes up has a lot to do with how much intelligence is on the signal.

A simple example of this is CB radio channels. The channels are spaced so that they are at least 10 KHz apart. This is so that one channel does not interfere with another channel. While no real intelligence is usually conveyed, the space it required. :)

Say we have 3 KHz sine wave modulating (simple AM) a radio channel. The bandwidth required would be the space necessary to contain the carrier and the two sideband signals at carrier+3KHz and carrier-3KHz. Overall we would need 7-8 KHz of bandwidth to ensure that the sidebands are not attenuated much. Generally, the same would be required on the receiver, to ensure that the signal can be recovered properly. (With AM you could get away with half that bandwidth on receive, because one sideband is redundant and can be discarded.)

Now, say this 3 KHz sine was was chopped into a series of different length tones. And say a tones length, and it's relationship to other tones, conveys intelligence (i.e. Morse Code). But we want the message to be sent in 1/2 the time. You could handle that in two ways. One is to simply cut each tone and tone space in half and send it out. Same intelligence in half the time. Or you could record the original tones and then speed up the playback by 2X. The tones and spaces will still be cut in length but the tone itself will double in frequency to 6 KHz. To completely encompass the signal now (carrier and both sidebands), you need twice the bandwidth. Yes, you only need it for half the time, but you still need it in order for the intelligence to be recovered.

Now take that example and modify it to include speech that ranges from say 300 to 2,000 Hz. Then increase the speed so that one minute of audio is transmitted in one second (60:1). Now all the audio will range in frequency from 1800 Hz to 12,000 Hz. This would increase the bandwidth requirements from 5-6 KHz to around 30 KHz. If you did this on a CB channel, you would cause interference across three channels.

Granted, that is a simple analog example using an antiquated mode of transmission. There are other transmission modes and data encoding that will help reduce the bandwidth required. But it is something that you need to consider.

See, this is what I'm after. Like saying the ABC's over the radio and having it go over the air like a yelp.
Yes, I understand what you are after. I'm just not sure I see a point in it yet. Have you taken that recording and slowed down the section where he recites "all 50 states in 1/4 of a second"? I'll bet they can't really fit it in the bandwidth capability of that recording.
 

poltergeisty

Deep Thinker
Joined
May 7, 2004
Messages
3,808
Location
RLG, Fly heading 053, intercept 315 DVV
Interesting. I know submarines burst out a signal to a sat. I just wonder what their bandwidth is? Maybe I need to focus instead on a A/D convertor?

I'm not sure what IC's would be used in such a project. My electronic knowledge goes a far as reading the Engineers Mini Notebook's from Forest M. Mims. :lol:
 

K7MEM

Member
Joined
Dec 16, 2013
Messages
341
Location
1158 W. Valley Circle, Ash Fork, AZ 86320-482
Interesting. I know submarines burst out a signal to a sat. I just wonder what their bandwidth is? Maybe I need to focus instead on a A/D convertor?

I'm not sure what IC's would be used in such a project. My electronic knowledge goes a far as reading the Engineers Mini Notebook's from Forest M. Mims. :lol:
When they are working with sat communication from a sub, I don't think they worry too much about bandwidth. They are operating very directional in the high GHz range. Using 100 MHz bandwidth, or more, wouldn't be a problem.

I have a copy of that book myself. It has some nice tidbits of information, so I keep it around. I always forget my log relationships, so it does come in handy.

I am pretty sure you have some computer experience, so a good learning direction might be to get something like this:

Easy8051 v6 Development System - Atmel 8051 (AT89, S51) Microcontroller Board - mikroElektronika

It's a development kit for the 8051 series of micro-controllers. The development board comes with a wide variety of interface elements. I picked up an older version at a ham fest for cheap. It comes with all the necessary software and cables. Plus, mine came with two extra controller chips (2-AT89s8253, 1-AT89s2051). The development board has and A/D, a D/A, temperature sensor, displays, etc.. Lot's of thing to learn from. Of course, initially you learn to just read switch closures and light LEDs, but you don't have to stop there.

I find these development kits at ham fests all the time. You don't have to be a ham to go to a ham fest. I have a TMS320C2X DSP development kit from TI and another one for the Motorola MC68HC05. They may be a little dated, but still provide a lot of learning.
 
Status
Not open for further replies.
Top