High-fidelty audio-band tone burst - Page 1
Forum section: electronics / projects

author: jmarkwolf
author_rank: Regular Contributor
timestamp: September 11, 2017, 12:30:23 pm
content: I need to generate a high-fidelity tone burst, of selectable frequency (400Hz, 900Hz & 1600Hz) and varying modulation (sweeping between 1.5Hz and 20Hz).The tone burst part is easy, but I’m getting an artifact tone that I believe is the “envelope” formed by the low frequency modulation (sweeping between 1.5Hz and 20Hz). It should be below human hearing, but plainly audible none the less. I'm also getting a “click” sounding a lot like a “mic key click”, presumably at the actual start and stop of the tone.Is it possible to start and stop a pure tone burst without the extra artifact? Will simple high pass filtering do it?It may not need to be a sine tone, a simple digital tone may be fine, but this low-frequency content is problematic for me.Also, I'm much more the digital designer than analog. Can you recommend inexpensive (or free) tools for investigating.

author: JanJansen
author_rank: Frequent Contributor
timestamp: September 11, 2017, 02:03:38 pm
content: For the click you need to reset your sine wave.For the artifact : dont you have interpolation ?, and how big is your sinus lookuptable.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 11, 2017, 02:50:03 pm
content: No, in theory it is not possible (if you absolutely need the fast ON/OFF turn of the 900 Hz signal).Any square signal will generate an infinite spectrum of odd frequencies. So, if you turn ON/OFF the 900Hz sinusoidal audio with a frequency of, let's say 15 Hz, then the 3rd harmonic of 15 Hz will be 45 Hz, perfectly audible, and so on. Same for the 1.5Hz, the harmonics will be all over the audio spectrum, at 3 Hz apart and descending amplitude.To get rid of the "click" sound, you need to reduce the harmonics, which in this case will be to use a gentle start/stop of the 900Hz (like a fade in fade out), but even this is not free of higher spectrum harmonics.The best approach will be to use a sinusoidal waveform for the 1.5-20 Hz modulator signal instead of an ON/OFF approach.

author: jmarkwolf
author_rank: Regular Contributor
timestamp: September 11, 2017, 04:15:49 pm
content: Thanks for the input everybody.So far I'm just playing with Multisim and some function generators, and a simulated speaker. I think I need to play with an honest-to-goodness audio tool and some easily modifiable wave shaping.Then if I can get the three tones right, just capture some samples, and play them back at the correct intervals.Whaddya think?

author: RoGeorge
author_rank: Super Contributor
timestamp: September 11, 2017, 05:13:20 pm
content: If all you want is to listen to the sound, or to create a file out of it, then there are many free tools that can generate modulated audio using the sound card.Google told me this one: Daqartahttp://www.daqarta.com/dw_gen.htmFree, in 2 minutes it is possible to have the waveform you described.By fooling around with it, in 10 minutes it was possible to get some alien voices out of it, by activating a frequency sweep simultaneous with a FM and AM at different rates, all 100% modulated by a 2 Hz pulse. Very funny sounds. It has many features, can work with 32 bits audio, has MIDI, and so on, see the help.Another tool that can help would be an audio editor, like Audacity. Also free.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: September 11, 2017, 05:34:09 pm
content: "Click" is just an ultra-wide bandwidth signal. To minimise occupied spectrum BW you need to apply an envelope function to the carrier pulse (your tone.)It does not matter if carrier is 450MHz, 400Hz or 1.5Hz, the problem is the same - when the carrier turns on (start of OOK pulse) or turns off you get an infinitely wide spectral splash. Your occupied spectrum should be just a few Hz to few dozens Hz to get really clean tone.Some call itpulse shaping, some -window function.High pass filtering won't help because the "click" spectrum extends in both directions from your carrier (tone) - up and down.You need very tight bandpass filter.Which is the same thing as applying the pulse shaping in the first place.Starting constant amplitude sine tone from zero crossing and ending it on another zero crossing will still generate infinitely wide spectrum.CheersLeo

author: DaJMasta
author_rank: Super Contributor
timestamp: September 11, 2017, 05:35:16 pm
content: It's also worth mentioning that you will still be able to hear a 1.5Hz to 20Hz tone that is a perfect sine (no higher harmonics) and is started in phase with the carrier (no zero-crossing noise on start or end), it just won't sound like a tone, it will sound like a.... throbbing, maybe?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: September 11, 2017, 05:47:43 pm
content: You should look at the old General Radio Corp. 1396-A Tone Burst Generator. It's not really a generator at all, but a switch for an external generator with zero crossing circuits. The idea may not be suitable to what you're doing, but a read of the manual might prove useful.http://www.ietlabs.com/pdf/Manuals/GR/1396-A%20Tone%20Burst%20Generator.pdf

author: jmarkwolf
author_rank: Regular Contributor
timestamp: September 11, 2017, 06:03:23 pm
content: Thanks again everyone.I was beginning to wonder if synchronizing the zero crossing of the tone and the envelope might be part of my problem.That Daqarta program might be just the ticket for making it easy to experiment with some wave shaping, etc.

author: RoGeorge
author_rank: Super Contributor
timestamp: September 11, 2017, 08:53:55 pm
content: The general rule is thatthe sharper an edge is, the more strong the harmonics you will be.Even if you synchronize the modulation signal with a zero crossing of the the 900 Hz carrier, you will still have harmonics.As pointed before by DaJMasta, a sub 20Hz signal is perfectly audible on average speakers (not laptop or phone speakers). If you use it to modulate a 900Hz, it will be even easier to hear the sub 20 Hz tone.It's impossible to modulate 100% a 900Hz sinus with a 1.5-20Hz square without hearing the 1.5-20Hz and all their harmonics. Even if the 1.5-20Hz is sinusoidal, you will steal hear it as another tone together with the the 900Hz tone.Just as a curiosity, why do you need such a signal, what exactly do you want to make, or test with it?

author: David Hess
author_rank: Super Contributor
timestamp: September 11, 2017, 10:54:40 pm
content: Quote from: jmarkwolf on September 11, 2017, 06:03:23 pmThere is no phase alignment which will solve this problem. A sine tone burst results in more low frequency splatter and a cosine tone burst results in more high frequency splatter.https://www.pa.msu.edu/acoustics/turnon.pdfYou can either filter the modulating waveform to make a more gradual turn-on and turn-off or filter the result. The later could be done easily using a switched capacitor bandpass filter.

author: jmarkwolf
author_rank: Regular Contributor
timestamp: September 12, 2017, 12:05:56 pm
content: Quote from: RoGeorge on September 11, 2017, 05:13:20 pmI installed DAQARTA and played with it last evening. It's quite a comprehensive program and I was able to get some nice clean tone bursts out of it.It's free for 30 days, then "some" of the features "gray out" and stop working.I haven't figured out how to "capture and freeze" the waveforms yet (oscilloscope function), to get a good look at them yet. The waveform only exists while the signal generator is running, and I was unable to achieve triggering to get a good look at the wave shaping, but I'm sure it will be very educational. It makes it easy to play with the wave shaping to achieve the desired sound. Then I should be able to recreate what I need either through hardware circuitry or selective WAV file playback.I'll need to figure out the I/O features of my sound card as well.Might look at Audacity also.My ultimate goal is to generate a "warning warble" in a vehicle, the frequency and modulation of which will denote varying degrees of "urgency".Thanks for the tips everyone.