record and slow play instead of mixing down to audio?

Hi, a normal radio mixer would mix the LO with the RF to produce audio (assumming direct conversion)
I wonder, for KHz emmissions (LF or VLF) can one record these somehow and play them in very slow speed, in order to hear their audio ?

I wonder, for KHz emmissions (LF or VLF) can one record these somehow and play them in very slow speed, in order to hear their audio ?

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Replaying the record with respective low rate will produce audible signals. But what is it good for? What do you expect to hear?

neazoi said:

Hi, a normal radio mixer would mix the LO with the RF to produce audio (assumming direct conversion)...

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LO mixed with RF will give IF that is amplified using narrow bandpass (455kHz) filters. That gives better resolution.

Play them in slow speed can mean many things; you may simply get unintelligible sound. You need to pad small parts with blanks (or repeats).

FvM said:

Replaying the record with respective low rate will produce audible signals. But what is it good for? What do you expect to hear?

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Hm... would that be possible for voice? would that be possible for morse code?

Hm... would that be possible for voice? would that be possible for morse code?

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Morse yes, it drops the pitch and the dot/dash speed of the carrier, bringing it into the audio range.
Voice no, it drops the carrier so you hear a tone but the modulation rate is also dropped so in most cases the voice would go sub-sonic.

Many years ago I developed high speed tape duplicating machines, they could copy a C90 cassette tape (both sides) in about 45 seconds but it suffered two major problems. One was 'thunder' sounds which were traced to slowed down tape static discharges, the other was a constant whining noise which was traced to pick up of a radio station on 198KHz. When slowed to about 1/60 speed it produced a recording at 3.3KHz but no modulation was noticed. If you assume the highest modulation frequency of a LW AM transmitter is about 9KHz, when slowed down it would only be 150Hz and the highest voice pitch about 50Hz.

Brian.

betwixt said:

Morse yes, it drops the pitch and the dot/dash speed of the carrier, bringing it into the audio range.
Voice no, it drops the carrier so you hear a tone but the modulation rate is also dropped so in most cases the voice would go sub-sonic.


Brian.

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Wouldn't the morse tone also decrease in the subsonic range in frequency?

If you record the RF of an AM radio station then slow it down when you play it back you will hear a constant tone (the carrier) slowly increasing and decreasing its loudness (the modulation). No audio.
To an FM station you will also hear a tone (the carrier) but you probably will not hear the slowly changing frequency (the modulation). No audio.

Wouldn't the morse tone also decrease in the subsonic range in frequency?

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Under normal circumstances, no. Very few transmissions use AM with a keyed audio tone, the whole carrier is keyed on and off. If you slow the carrier frequency down so it is audible, the Morse speed is also reduced of course so it would probably be far too slow to read without needing bathroom visits and sleep breaks between words!

For example 12 words per minute on 1.8MHz dropped to say 1KHz audio would play at about one word per 3 minutes! Even I, with rusty ears can read Morse at that speed :lol:

Brian.

betwixt said:

Under normal circumstances, no. Very few transmissions use AM with a keyed audio tone, the whole carrier is keyed on and off. If you slow the carrier frequency down so it is audible, the Morse speed is also reduced of course so it would probably be far too slow to read without needing bathroom visits and sleep breaks between words!

For example 12 words per minute on 1.8MHz dropped to say 1KHz audio would play at about one word per 3 minutes! Even I, with rusty ears can read Morse at that speed :lol:

Brian.

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I got this idea from people that record EM waves of the planets and then transform them to the audio range to hear the "sounds" of the planets. However there is a mixing down I suppose.

It might seem of no interest but many discoveries have been discovered from ideas of no interest at the past.

I wonder what would the advantages be in such a HF RF record and slow down system, over an ordinary mixing system (if there are any).
Also I wonder how could this HF RF record be done with today's technology?
I am just curious.

Some astronomical bodies emit radiation which can be treated as AM and recovered through a simple diode detector. Pulsars for example can emit signals pulsed at quite low frequencies.

The principle used these days to 'slow down' a signal is to digitize it at a fast rate and store the samples in RAM then read the RAM back through a DAC at slower speed. The only practical use for such a system over normal radio I can think of is to recover high speed Morse bursts used in meteor scatter experiments. For that, a Morse message is recorded and transmitted at high speed as the radio link may only be open for a second or two. A receiver records it and slows it down so the message can be recovered at normal speed. It isn't the same as actually slowing the carrier itself to make it audible though.

A similar system is used on some telecoms protocols. The phone continuously digitizes the microphone output but sends it periodically as high speed data bursts. It's a method of increasing the channel capacity by sharing time slots with other users. The bursts are slowed down and stitched together at the receiving end so they sound uninterrupted. Look up "Time Compression Multiplexing" for more info.

Brian.

betwixt said:

Some astronomical bodies emit radiation which can be treated as AM and recovered through a simple diode detector. Pulsars for example can emit signals pulsed at quite low frequencies.

The principle used these days to 'slow down' a signal is to digitize it at a fast rate and store the samples in RAM then read the RAM back through a DAC at slower speed. The only practical use for such a system over normal radio I can think of is to recover high speed Morse bursts used in meteor scatter experiments. For that, a Morse message is recorded and transmitted at high speed as the radio link may only be open for a second or two. A receiver records it and slows it down so the message can be recovered at normal speed. It isn't the same as actually slowing the carrier itself to make it audible though.

A similar system is used on some telecoms protocols. The phone continuously digitizes the microphone output but sends it periodically as high speed data bursts. It's a method of increasing the channel capacity by sharing time slots with other users. The bursts are slowed down and stitched together at the receiving end so they sound uninterrupted. Look up "Time Compression Multiplexing" for more info.

Brian.

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Interesting!
In what kind of technologies can someone today "record" an HF carrier (if he can) without mixing it down?

Most simple microcontrollers can digitize up to a few tens of KHz.
Standard flash video converters up to a few tens of MHZ.
Special ADCs can work beyond 100MHz.

There is usually a trade off between speed and accuracy, especially in microcontrollers which generally use successive approximation as the conversion method.

The problem is just as much with the data storage though. The faster you digitize the more storage you need for a given time period. ADCs and memory also need some signals to to control and coordinate them so that can also be a limiting factor. For example, with 8-bit data, which is adequate for most things (more bits = more accuracy) to digitize a 1.8MHz signal at MINIMUM recoverable rate (Nyquist limit) would need almost 30Mbits of memory per second. To be able to use the stored waveform for practical use, a faster rate would be needed, preferably more than 100 samples per cycle so the storage would need to be about 50 times bigger.

Again I stress there is little point in 'slowing down' a carrier to reduce it's frequency unless you want a detailed analysis of the wave shape. Because the modulation is also slowed down by the same factor it makes it impractical to use for voice or data recovery. The advantage of down-mixing is the frequency is dropped without losing the amplitude envelope or modulation index.

Brian.

betwixt said:

... Pulsars for example can emit signals pulsed at quite low frequencies. ...

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Crab nebula a nearby pulsar (rotating neutron star)- about 20-30 km in size and emits strong radiation in pulses 30.2 Hz (30.2 pulses per second). See https://en.wikipedia.org/wiki/Crab_Nebula

It is an extremely powerful source but cannot be detected in a simple radio- you will need a radio telescope to see the pulses.

Recently voyager spacecraft left the solar system; the signal became so weak that special tools were needed for communications.

The sun also emits radio waves and that also can be seen in a radio telescope (I do not know whether it can be seen in a regular radio).

The sun also emits radio waves and that also can be seen in a radio telescope (I do not know whether it can be seen in a regular radio).

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It is so strong it causes satellite 'black outs' when it aligns with satellites and the receiving dish. So much interference it can swap the LNB front end. Some of the background 'hash' is also from cosmic sources, particularly the Sun due to it's proximity. Jupiter can also be picked up with modest radio equipment and a small dish or even high gain Yagi arrays.

Brian.