I want to modulate digital data into audio. Then communicate it through any audio channel and demodulate at the destination from audio to data again. To do this I hope to use computer sound card and software modem without using any hardware implementation. In the internet, I found that this can be through the technique called Audio Frequency-Shift Keying(AFSK). I want to know that can I obtain bit rate more than 1200bps from AFSK and if it is no what the reason behind that this limitation.
Is there any technique efficient than AFSK for this purpose ?
The most common currently-used form of AFSK is the Bell202 modem at 1200 baud. There are a few other standards which also use 1200 baud, and some that run at less than 1200 bits per second, but none that I know of that run greater than 1200.
However, as far as I know, there's no reason you couldn't write a software modem to transmit and receive at a higher baud rate. Bell202 uses bit stuffing (allowing the data stream to use the same tone no more than 5 bits in a row) to help keep the transmitter and receiver from falling out of sync with each other, so a higher baud rate might require bit stuffing at a lower threshold (every 4 or 3 bits).
Another consideration is that the sound cards you're using should use a sampling rate equal to or a multiple of the baud rate you choose. This is one of the reasons 1200 baud is so common, as 1200Hz and 48000Hz are common sample rates with audio hardware.
So 1200 baud isn't a limit. It's just a standard.
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I'm trying to understand how to implement audio playback from scratch on attiny85. The goal is to play a short sound (cat meows, so i want it to remain recognizable) from an array representing strength of audio signal sampled at fixed interval.
As far as i understand, signal strength is linearly mapped to voltage of analogue audio signal. As far as I know, audio cards are Digital to Analogue Converters, but attiny85 probably doesn't have that.
I'm curious if I can use pwm to play the sound back. Since pwm changes average voltage by changing duty cycle of alternating high and low phases of signal, it most likely would result in the drop of audio quality. Wav sampling rates can differ between 1 HZ and 4.3 GHz according to google. Attiny85 has internal clock with frequency up to 8MHz (which I hope is same for it's pwm generator).
Considering reconfiguring the timer and pwm settings as well as looping in the array, what is the maximum sampling rate of audio i can reliably play? And should i even try to do it with pwm, or there are better options?
Given a system clock of 8 MHz, you can use PWM to generate mono (single-channel) audio.
Consider a PWM period of 1000 clocks, giving you about 10 bit resolution. The sample rate will be 8000 Hz then, which gives you some kind of lo-fi audio.
If you reduce your signal resolution to 8 bits, you'll get 8 MHz / 28 = 31.25 kHz sample rate. This gets near hi-fi.
Synchronize your sample output with the PWM generator, and use an appropriate analogue filter.
Many years ago I built a digital door bell with a sample rate of 8 kHz and 8 bit samples. It played nice sounds in the quality of telephones. The microcontroller was a 8051 derivative and it used an R-2R ladder as DAC.
A simpel sinus can be generated by using a 50% PWM signal and varying the frequency. Given some filtering effect through the speaker, it would mimik a single tone audio signal.
Making more advanced tones (needed for natural sound) quickly gets more complicated and the duty cycle of the signal can also be used to trick the human ear into hearing harmonics. Check out the arduino function tone() for some inspiration.
Be carefull when connecting a small speaker to the Arduino, preferably a transistor/buffer/small amplifyer should be place between the Arduino and the speaker.
My goal is to record audio using an electret microphone hooked into the analog pin of an esp8266 (12E) and then be able to play this audio on another device. My circuit is:
In order to check the output of the microphone I connected the circuit to the oscilloscope and got this:
In the "gif" above you can see the waves made by my voice when talking to microphone.
here is my code on esp8266:
void loop() {
sensorValue = analogRead(sensorPin);
Serial.print(sensorValue);
Serial.print(" ");
}
I would like to play the audio on the "Audacity" software in order to have an understanding of the result. Therefore, I copied the numbers from the serial monitor and paste it into the python code that maps the data to (-1,1) interval:
def mapPoint(value, currentMin, currentMax, targetMin, targetMax):
currentInterval = currentMax - currentMin
targetInterval = targetMax - targetMin
valueScaled = float(value - currentMin) / float(currentInterval)
return round(targetMin + (valueScaled * targetInterval),5)
class mapper():
def __init__(self,raws):
self.raws=raws.split(" ")
self.raws=[float(i) for i in self.raws]
def mapAll(self):
self.mappeds=[mapPoint(i,min(self.raws),max(self.raws),-1,1) for i in self.raws ]
self.strmappeds=str(self.mappeds).replace(",","").replace("]","").replace("[","")
return self.strmappeds
Which takes the string of numbers, map them on the target interval (-1 ,+1) and return a space (" ") separated string of data ready to import into Audacity software. (Tools>Sample Data Import and then select the text file including the data). The result of importing data from almost 5 seconds voice:
which is about half a second and when I play I hear unintelligible noise. I also tried lower frequencies but there was only noise there, too.
The suspected causes for the problem are:
1- Esp8266 has not the capability to read the analog pin fast enough to return meaningful data (which is probably not the case since it's clock speed is around 100MHz).
2- The way software is gathering the data and outputs it is not the most optimized way (In the loop, Serial.print, etc.)
3- The microphone circuit output is too noisy. (which might be, but as observed from the oscilloscope test, my voice has to make a difference in the output audio. Which was not audible from the audacity)
4- The way I mapped and prepared the data for the Audacity.
Is there something else I could try?
Are there similar projects out there? (which to my surprise I couldn't find anything which was done transparently!)
What can be the right way to do this? (since it can be a very useful and economic method for recording, transmitting and analyzing audio.)
There are many issues with your project:
You do not set a bias voltage on A0. The ADC can only measure voltages between Ground and VCC. When removing the microphone from the circuit, the voltage at A0 should be close to VCC/2. This is usually achieved by adding a voltage divider between VCC and GND made of 2 resistors, and connected directly to A0. Between the cap and A0.
Also, your circuit looks weird... Is the 47uF cap connected directly to the 3.3V ? If that's the case, you should connect it to pin 2 of the microphone instead. This would also indicate that right now your ADC is only recording noise (no bias voltage will do that).
You do not pace you input, meaning that you do not have a constant sampling rate. That is a very important issue. I suggest you set yourself a realistic target that is well within the limits of the ADC, and the limits of your serial port. The transfer rate in bytes/sec of a serial port is usually equal to baud-rate / 8. For 9600 bauds, that's only about 1200 bytes/sec, which means that once converted to text, you max transfer rate drops to about 400 samples per second. This issue needs to be addressed and the max calculated before you begin, as the max attainable overall sample rate is the maximum of the sample rate from the ADC and the transfer rate of the serial port.
The way to grab samples depends a lot on your needs and what you are trying to do with this project, your audio bandwidth, resolution and audio quality requirements for the application and the amount of work you can put into it. Reading from a loop as you are doing now may work with a fast enough serial port, but the quality will always be poor.
The way that is usually done is with a timer interrupt starting the ADC measurement and an ADC interrupt grabbing the result and storing it in a small FIFO, while the main loop transfers from this ADC fifo to the serial port, along the other tasks assigned to the chip. This cannot be done directly with the Arduino libraries, as you need to control the ADC directly to do that.
Here a short checklist of things to do:
Get the full ESP8266 datasheet from Expressif. Look up the actual specs of the ADC, mainly: the sample rates and resolutions available with your oscillator, and also its electrical constraints, at least its input voltage range and input impedance.
Once you know these numbers, set yourself some target, the math needed for successful project need input numbers. What is your application? Do you want to record audio or just detect a nondescript noise? What are the minimum requirements needed for things to work?
Look up in the Arduino documentartion how to set up a timer interrupt and an ADC interrupt.
Look up in the datasheet which registers you'll need to access to configure and run the ADC.
Fix the voltage bias issue on the ADC input. Nothing can work before that's done, and you do not want to destroy your processor.
Make sure the input AC voltage (the 'swing' voltage) is large enough to give you the results you want. It is not unusual to have to amplify a mic signal (with an opamp or a transistor), just for impedance matching.
Then you can start writing code.
This may sound awfully complex for such a small task, but that's what the average day of an embedded programmer looks like.
[EDIT] Your circuit would work a lot better if you simply replaced the 47uF DC blocking capacitor by a series resistor. Its value should be in the 2.2k to 7.6k range, to keep the circuit impedance within the 10k Ohms or so needed for the ADC. This would insure that the input voltage to A0 is within the operating limits of the ADC (GND-3.3V on the NodeMCU board, 0-1V with bare chip).
The signal may still be too weak for your application, though. What is the amplitude of the signal on your scope? How many bits of resolution does that range cover once converted by the ADC? Example, for a .1V peak to peak signal (SIG = 0.1), an ADC range of 0-3.3V (RNG = 3.3) and 10 bits of resolution (RES = 1024), you'll have
binary-range = RES * (SIG / RNG)
= 1024 * (0.1 / 3.3)
= 1024 * .03
= 31.03
A range of 31, which means around Log2(31) (~= 5) useful bits of resolution, is that enough for your application ?
As an aside note: The ADC will give you positive values, with a DC offset, You will probably need to filter the digital output with a DC blocking filter before playback. https://manual.audacityteam.org/man/dc_offset.html
I have tried to send 12-bit audio to be listened to in real time through the HC05 SPP bluetooth module hooked up to an arduino and DAC over serial with a python RFCOMM socket. I have since learned that Serial Port Protocol is not very great at all for this purpose due to its low bandwidth. I figured I could definitely send the data and then play it out through a DAC, but I doubt an arduino would hold an array the size of a WAV file and maybe not even an mp3 file, but that would defeat the purpose of controlling the audio (play,pause,rewind,etc) from my computer. Would it be more realistic and worthwhile to use an A2DP enabled bluetooth module? Or is it still possible to listen to acceptable quality 12-16 bit audio in real time with SPP? I have tried to use lower bit songs, adjusted baud rates for the arduino and HC-05 serial ports, and tried to adjust the magnitude of the values outputted by the DAC to the audio port and I still seem to get crackly audio. It seems the problem comes down to the low bitrate transfer speed of SPP, or am I wrong?
Is it realistic to stream 12-16 bit audio through SPP bluetooth in realtime?
Sure, at some awfully slow sample rate <= 8 kHz. You'd be better off sending 8-bit audio at a higher sample rate.
Would it be more realistic and worthwhile to use an A2DP enabled bluetooth module?
Yes, absolutely, without question. That's what it's designed for, as I mentioned in your other question.
Or is it still possible to listen to acceptable quality 12-16 bit audio in real time with SPP?
Acceptable is subjective. If it's just voice, you can get away with it. If you want reasonable audio quality for music, almost universally, no, it's not acceptable.
It seems the problem comes down to the low bitrate transfer speed of SPP, or am I wrong?
Without any code to inspect and debug, it's impossible to say what the specific problem is that you're referring to. Undoubtedly, the low bandwidth will not enable good quality audio anyway.
If you must continue to use SPP and simple codecs like PCM, at least use differential PCM to save a bit more bandwidth.
I am using python sockets to connect to a bluetooth HC-05 module with my PC. I want to send music to the HC-05 by converting a wav file to a string array which will later by converted to integers ranging from 0-65535 on an arduino. The arduino and HC-05 communicate via serial at 9600 baud. Then those ints will be passed into a DAC via I2C. I am wondering if there could be a memory issue sending an enormous number of strings from my PC. Is it possible the original quality of the sound will be distorted as a result of the different rates of sending/receiving data across the devices? Or will the sound signal just be delayed on the DAC?
The arduino and HC-05 communicate via serial at 9600 baud.
This is going to be too low to be usable for audio.
9600 baud gives you 7680 bits of data per second. At 16 bits per sample, you're looking at a sample rate of 481 Hz, which is too low for intelligible audio. It's barely even high enough to reproduce sound at all.
You need to:
Increase the baud rate. Ideally you'll want at least 57600 baud, for 46 kbit/sec of data. If higher baud rates are available, use them.
Use fewer bits for each sample. Using 4 bits for each sample at 56 kbaud will give you a respectable sample rate of 11.5 kHz. The audio will sound a little tinny at 4 bits, but it'll be intelligible.
I had a assignment for college where we needed to play a precompiled wav as integer array through the PWM and DAC. Now, I wanted more of a challenge, so I went out of my way and created a audio dac over usb using the micro controller in question: The STM32F051. It basically listens to my soundcard output using a wasapi loopback recorder, changes the resolution from 16 to 12 bit (since the dac on the stm32 only has a 12 bit resolution) and sends it over using usart using 10x sample rate as baud rate (in my case 960000). All done in C#.
On the microcontroller I simply use a interrupt for usart and push the received data to the dac.
It works pretty well, much better than PWM, and at a decent sample frequency of 48kHz.
But... here it comes.. When there is some (mostly) high pitch symphonic melody it starts to sound "wobbly".
Here a video where you can hear it: https://youtu.be/xD3uTP9etuA?t=88
I read up on the internet a bit about DIY dac's and someone somewhere (don't remember where) mentioned that MCU's in general have interrupt jitter. So may basic question is: Is interrupt jitter actually causing this? If so, are there ways to limit the jitter happening?
Or is this something entirely different?
I am thinking of trying to compact the pcm data send over serial (as said before, resolution of 12 bits, but are sent in packet of 2 8bits forming 16bits, hence twice the samplerate as the baud rate, so my plan is trying to shift 12 bits to the MSB and adding four bits of the next 12 bit value to the current 16 bit variable, hence only needing 12 transfers instead of 16 per 8 samples. Might read upon more efficient ways of compacting data for transport.), put the samples in a buffer and then use another timer that triggers at 48kHz for sending the samples to the dac. Would this concept work? Or would I just waste time?
For code, here is the project: https://github.com/EldinZenderink/SoundOverSerial