A Basic Limitation in Stereo FM Reproduction

One of the concepts which exist in modern, high-definition sound, is that Human Sound perception can take place between 20 Hz and 20kHz, even though those endpoints are somewhat arbitrary. Some people cannot hear frequencies as high as 20kHz, especially older people, or anybody who just does not have good hearing. Healthy, young children and teenagers can typically hear that entire frequency range.

But, way back when FM radio was invented, sound engineers had flawed data about what frequencies Humans can hear. It was given to them as data to work with that Humans can only hear frequencies from 30Hz to 15kHz. And so, even though Their communications authorities had the ability to assign frequencies somewhat arbitrarily, they did so in a way that was based on such data. (:1)

For that reason, the playback of FM Stereo today, using household receivers, is still limited to an audio frequency range from 30Hz to 15kHz. Even very expensive receivers will not be able to reproduce sound, that was once part of the modulated input, outside this frequency range, although other reference points can be applied, to try to gauge how good the sound quality is.

There is one artifact of this initial standard which was sometimes apparent in early receivers. Stereo FM has a pilot frequency at 19kHz, which a receiver needs to lock an internal oscillator to, but in such a way that the internal oscillator runs at 38kHz, but such that this internal oscillator can be used to demodulate the stereo part of the sound. Because the pilot signal which is actually part of the broadcast signal is ‘only’ at 19kHz, this gives an additional reason to cut off the audible signal at ‘only’ 15Khz; the pilot is not meant to be heard. But, way back in the 1970s and earlier, Electrical Engineers did not have the type of low-pass filters available to them which they do now, that are also known as ‘brick-wall filters’, or filters that attenuate frequencies above the cutoff frequency very suddenly. Instead, equipment designed to be manufactured in the 1970s and earlier, would only use low-pass filters with gradual ‘roll-off’ curves, to attenuate the higher frequencies progressively more, above the cutoff frequency by an increasing distance, but in a way that was gentle. And in fact, even today the result seems to be, that gentler roll-off of the higher frequencies, results in better sound, when the quality is measured in other ways than just the frequency range, such as, when sound quality is measured for how good the temporal resolution, of very short pulses, of high-frequency sound is.

Generally, very sharp spectral resolution results in worse temporal resolution, and this is a negative side effect of some examples of modern sound technology.

But then sometimes, when listeners with high-end receivers in the 1970s and before, who had very good hearing, were tuned in to an FM Stereo Signal, they could actually hear some residual amount of the 19kHz pilot signal, which was never a part of the original broadcast audio. That was sometimes still audible, just because the low-pass filter that defined 15kHz as the upper cut-off frequency, was admitting the 19kHz component to a partial degree.

One technical accomplishment that has been possible since the 1970s however, in consumer electronics, was an analog ‘notch filter’, which seemed to suppress one exact frequency – or almost so – and such a notch filter could be calibrated to suppress 19kHz specifically.

Modern electronics makes possible such things as analog low-pass filters with a more-sudden frequency-cut-off, digital filters, etc. So it’s improbable today, that even listeners whose hearing would be good enough, would still be receiving this 19kHz sound-component to their headphones. In fact, the sound today is likely to seem ‘washed out’, simply because of too many transistors being fit on one chip. And when I just bought an AM/FM Radio in recent days, I did not even try the included ear-buds at first, because I have better headphones. When I did try the included ear-buds, their sound-quality was worse than that, when using my own, valued headphones. I’d say the included ear-buds did not seem to reproduce frequencies above 10kHz at all. My noise-cancelling headphones clearly continue to do so.

One claim which should be approached with extreme skepticism would be, that the sound which a listener seemed to be getting from an FM Tuner, was as good as sound that he was also obtaining from his Vinyl Turntable. AFAIK, the only way in which this would be possible would be, if he was using an extremely poor turntable to begin with.

What has happened however, is that audibility curves have been accepted – since the 1980s – that state the upper limit of Human hearing as 20kHz, and that all manner of audio equipment designed since then takes this into consideration. This would include Audio CD Players, some forms of compressed sound, etc. What some people will claim in a way that strikes me as credible however, is that the frequency-response of the HQ turntables was as good, as that of Audio CDs was. And the main reason I’ll believe that is the fact that Quadraphonic LPs were sold at some point, which had a sub-carrier for each stereo channel, that differentiated that stereo channel front-to-back. This sub-carrier was actually phase-modulated. But in order for Quadraphonic LPs to have worked at all, their actual frequency response need to go as high as  40kHz. And phase-modulation was chosen because this form of modulation is particularly immune to the various types of distortion which an LP would insert, when playing back frequencies as high as 40kHz.

About Digital FM:

(Updated 6/24/2019, 14h50 … )

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Some observed progress in Lithium-Ion Batteries.

I have posted before about Lithium-Ion batteries. My relatively new Samsung Galaxy S9 Smart-Phone possesses one, and seems to have better battery performance overall, than my old Galaxy S6 did. Also, both these phones had a sensor chip which measures battery voltage. I use an Android app called GSam Battery Monitor Pro, to get occasional glimpses of what my battery is doing, and without such a hardware chip, the app would not be able to provide meaningful information. On my Galaxy S9 phone, the battery has a voltage of ~4.2V once fully charged, and I’m assuming that to plug the phone in just keeps it connected to a constant voltage source…

foxy_156094354928

On my previous, Galaxy S6, when fully charged, the battery had a voltage of ‘only’ ~3.7V. And so one strategy which Samsung could be using to increase battery life, would simply be to charge the same battery technology to a higher voltage. However, usually, doing so would only result in the battery catching fire. And so, some improvement in the actual design of the battery had to take place, so that it could be kept charged to 4.2V, and not suffer any immediate damage.

Dirk

 

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How to patch ‘kdeconnect’ to work under Debian / Stretch.

There exists an Android app named ‘kdeconnect’, which, when paired with the Debian / Stretch / Plasma 5.8 desktop widget by the same name, allows users to sync various features between their Linux desktop, and their Android device. The versions I’m presently using are:

  • Android app: 1.12.9
  • Linux package: 1.0.3~bpo9+0

Besides syncing certain basic messages, such as phone Notifications to the widget, this app allows for the desktop computer to browse directories on the Android device, which the user has authorized from his Android device – as long as the Linux desktop widget software has been patched! Below is another shot, of what this looks like when it’s working:

Screenshot_20190615_181900

The main observation about this is the fact, that it does not work out of the box. The reason for this is the fact that the Linux widget is out-of-date, as a backport. The Linux-based software tries to use an SSH-FS Mount, that specifies ‘DSA’ as its crypto-algorithm. DSA is an outdated, insecure protocol, for which reason the application framework of Android no longer supports it! Android will demand that RSA be used as a minimum.

And so, due to this initial incompatibility, the SSH-FS Mount, which creates a virtual file system in the user’s home directory, in a hidden sub-directory, fails, with an error message to the user that doesn’t seem helpful. This error message simply complains that certain files and folders could not be found, that are supposed to exist remotely, from the Android device.

And so at first glance it might seem like an unsolvable problem. But as it happens, with this exact version of the Linux package, there is a fix, which I’ve been using for months. In the past I wanted to keep this patch to myself, out of fear that my readers might botch this delicate surgery. But I’ve had a change of heart, in that I want everybody to benefit from this app, even if they are using an outdated version of the Linux software. If the reader has the courage to perform this surgery, then the following is for you:

Continue reading How to patch ‘kdeconnect’ to work under Debian / Stretch.

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Latest Android Update Breaks ‘kdeconnect’ on Debian Stretch (Already Resolved).

One of the apps which I have installed on my Android phone, is called ‘kdeconnect’, and I’ve blogged about it before. This is an app that allows a compatible Linux widget to sync certain data with the smart-phone.

Screenshot_20190216_130006

(Screen-Shot from some earlier version of this app, which did not constrain the available directories.)

The version which I have installed on the Debian / Stretch computer I name ‘Phosphene’, is 1.0.3~bpo9+0 . I actually needed to patch this package, so that for the following few months, it was able to browse the file-system of my phone, specifically, directories which I authorized on the phone app, from my Linux computer.

Well the Android companion to this app has just received an update through Google Play. This update broke the ability of my Linux computer to mount the remote file system – i.e., to browse any directories on the phone.

(Update at 18h25 : )

But what seems to have happened is that two updates were pushed to my phone in rapid succession, the second of which put the Android app version to 1.12.9 . The reason for which I’m inferring this, is the fact that this remote mounting of the phone’s chosen directories works now, with no actual intervention from me:

Screenshot_20190615_181900

The detail of this experience which puzzles me, is the thought that I had in fact been testing v1.12.9, when I first reported the app as broken… :-?

However, this ‘broken’ result can also occur, just because of faulty communication between the two devices.

Dirk

 

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