47
votes
Accepted
Why can so little digital information be stored on a cassette tape?
I had heard that tape is still the best medium for storing large amounts of data.
well, "best" is always a reduction to a single set of optimization parameters (e.g. cost per bit, ...
- 32k
11
votes
Accepted
Why do we integrate the modulating function in FM?
You need to integrate the modulating signal because frequency is the time derivative of phase. Therefore, the typical relationship from introductory calculus holds:
$$
\phi_i(t) = \int_{-\infty}^{t} \...
- 24.7k
11
votes
Accepted
Why do digital modulation schemes (in general) involve only two carrier signals?
Ah! They don't generally involve only two degrees of freedom. The ones you've been looking at mostly have, but that's not because they're the only ones used or taught.
Chances are you'll meet one or ...
- 32k
8
votes
Why can so little digital information be stored on a cassette tape?
High-capacity tape formats use helical scanning. Here I look at why that gives a higher capacity than linear recording with four tracks (one stereo track on each side) like in a compact cassette (C-...
- 13.6k
8
votes
I'm missing something when it comes to understanding FM
In addition to the perspective offered in @Ash's answer, Claude Shannon touches on FM in his paper "Communications in the Presence of Noise" (Proceedings of the IRE, January 1949). He ...
- 12.9k
7
votes
FSK and IQ modulation
What is the advantage of performing the FSK using IQ modulation?
You only need one RF oscillator operating at a single frequency, instead of having 2 (or more in the case of M-ary FSK) oscillators ...
- 121
6
votes
Accepted
Recommendation for good ressources on digital processing and D-QPSK
Wow, I'm honored by Matt L. doing what I'm often doing: Referring people to GNU Radio.
The project actually has a list of recommended literature, but I don't know how well that'd fit you. It's ...
- 32k
6
votes
Accepted
Difference between FFSK and FSK modulation schemes
Fast FSK is just another term for minimum-shift keying (MSK). In the modern literature, the term fast FSK has been almost completely replaced by MSK.
There is a very good tutorial paper on MSK by S. ...
- 91.2k
5
votes
How does the RaspberryPi radio hack convert digital GPIO signals to an FM signal?
First, the answer to why you do not see a negative voltage is that the output being digital will range from 0 to the maximum digital voltage at the output (+Vs). This will have a DC offset of +Vs/2 ...
- 53.8k
5
votes
Accepted
Quadrature modulation for FM and AM
Both amplitude and frequency modulated radio signals nowadays use quadrature modulation and demodulation as a mean to transfer and receive radio signals.
This question is ill-defined – a signal ...
- 32k
5
votes
What is the mixed numerology in OFDM
In general, it means several waveforms will coexist in 5G ecosystem.
In the context of LTE-based 5G (up to now), several subcarrier spacings (hence CP) coexist. The first and foremost motivation for ...
- 6,650
5
votes
Why can so little digital information be stored on a cassette tape?
You have already selected your answer, but I would like to put a few more lines.
First of all, objecting to Marcus, I think that your first assumption is correct! We can store one hour of almost CD ...
- 28.4k
5
votes
I'm missing something when it comes to understanding FM
Large modulation indexes are preferred because the signal-to-noise ratio has a cubic dependence on modulation index ($\beta$).
$$SNR_{FM}=3\beta^2(\beta+1) CNR$$
Where CNR is the carrier-to-noise ...
- 945
4
votes
Accepted
Simplified equations for FM (frequency modulation) synthesis and PM (phase modulation)?
If $t$ is time, $s(t)$ is the (appropriately scaled) signal, $\omega_0$ is the angular frequency, and $\phi_0$ is a phase offset, then phase modulation is
$$ t \mapsto \sin(\omega_0 t + \phi_0 + s(t)) ...
4
votes
FSK and IQ modulation
There are many ways of demodulating the FSK signal
Indeed!
What is the advantage of performing the FSK using IQ modulation?
Depends. Generally, IQ is the only shape you have your signal in, so ...
- 32k
4
votes
Accepted
How to convert sampled real audio signal to it's IQ components for IQ modulation?
I’ll answer the question I think you’re asking here:
If you have a real valued signal that came right out of an A/D and you’d like to recover signal phase, you can use the Hilbert transform to ...
- 863
4
votes
Accepted
How to estimate the frequency characteristics of a nonlinear chirp?
How about just using $\dfrac{d\phi}{dt}$ to compute the instantaneous frequency?
GNU Octave code (I just made up a sample rate of 2 Msps):
...
- 2,710
4
votes
Accepted
Positive slope of BER with a greater frequency deviation?
This could happen as discriminator gain is increased with a filter discriminator approach since in many of those approaches the gain would be maximum and linear for small signals only and then the ...
- 53.8k
4
votes
Why can so little digital information be stored on a cassette tape?
Typical cheap cassette tape recorders and players in the 1970's used audio circuitry that did not have a completely flat frequency response and added a lot of phase distortion (mostly inaudible to ...
- 35.6k
3
votes
Accepted
Frequency modulation of $m(t)$ in MATLAB
Integration was incorrect. I was integrating it with $dx= 1$. Doing m(j)*1/Fsoperation in the for loop, I have made $dx= 1/Fs$ now.
As can be seen, frequency ...
- 65
3
votes
Accepted
How does the RaspberryPi radio hack convert digital GPIO signals to an FM signal?
I just skimmed
https://github.com/SaucySoliton/PiFmRds/blob/master/src/pi_fm_rds.c#L454
and from what that code looks like, it initializes a clock generator to run at an adjustable clock.
Then, it ...
- 32k
3
votes
Impact of modulation index in FM
Assuming an FM modulation of a sine wave of frequency $f_m$. Theoretically, the modulated signal is an infinite sum of cosines at the frequencies $f_c + nf_m$ ($n\in \mathbb{Z}$) which the amplitudes ...
- 178
3
votes
difference between GMSK (Gaussian Minimum shift keying) and GFSK (Gaussain frequency shift keying)
With GMSK, the frequency modulation index is 0.5. With GFSK, the frequency modulation index is larger than 0.5 (the signaling tones are farther apart).
The receiving techniques used for GMSK as an FSK ...
- 2,710
3
votes
Biquad cookbook formula for broadcast FM de-emphasis?
Taken from this excellent RDS tutorial, and original source:
De-emphasis filter, $H(s) = \frac {1} {RC.s + 1}$, implemented as IIR filter via a bilinear transform:
...
- 473
3
votes
Biquad cookbook formula for broadcast FM de-emphasis?
You could also implement a 2nd order IIR filter which has response of 1st order filter by using some other TF but one used in RBJ's cookbook.
Here's one way to do it using MZT (C++).
...
- 927
3
votes
Accepted
FM demodulation with arctan
The $\arctan2()$ function has a jump at $\pm\pi$ which will cause problems when trying to compute a derivative.
Since you're using the first difference discrete derivative approximation to compute $d\...
- 2,710
3
votes
How to isolate overlapping FM signals?
What if they did all use the same carrier?
What you're describing is a single-frequency network (SFN). These are in common use for things that are not stupid FM broadcast.
The whole truth is that ...
- 32k
3
votes
Accepted
About phase unwrapping
The problem comes from sampling the phase. For the given phase function, the changes in the phase from one value $t_i$ to the next one $t_{i+1}$ can become too large, so phase unwrapping will not work ...
- 91.2k
3
votes
Homodyne and heterodyne detection
I'll assume AM DSB modulation for simplicity, but the same idea applies to quadrature modulation. I'll assume that the received signal has been properly bandpass filtered. I'll also ignore noise and ...
- 15.4k
3
votes
Accepted
FM synthesis algorithm is not very efficient
I think you're overthinking the integrator:
Since your $s$ is a discrete signal in itself (you're doing DSP!!), the integral $S(T) = \int\limits_0^T s(t) \,\mathrm dt$ of it at any time $T$ actually ...
- 32k
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