## Some realizations about Digital Signal Processing

One of the realizations which I’ve just come across recently, about digital signal processing, is that apparently, when up-sampling a digital stream twofold, just for the purpose of playing it back, simply to perform a linear interpolation, to turn a 44.1kHz stream into an 88.2kHz, or a 48kHz stream into a 96kHz, does less damage to the sound quality, than I had previously thought. And one reason I think this is the factual realization that to do so, really achieves the same thing that applying a (low-pass) Haar Wavelet would achieve, after each original sample had been doubled. After all, I had already said, that Humans would have a hard time being able to hear that this has been done.

But then, given such an assumption, I think I’ve also come into more realizations, of where I was having trouble understanding what exactly Digital Signal Processors do. It might be Mathematically true to say, that a convolution can be applied to a stream after it has been up-sampled, but, depending on how many elements the convolution is supposed to have, whether or not a single DSP chip is supposed to decode both stereo channels or only one, and whether that DSP chip is also supposed to perform other steps associated with playing back the audio, such as, to decode whatever compression Bluetooth 4 or Bluetooth 5 have put on the stream, it may turn out that realistic Digital Signal Processing chips just don’t have enough MIPS – Millions of Instructions Per Second – to do all that.

Now, I do know that DSP chips exist that have more MIPS, but then those chips may also measure 2cm x 2cm, and may require much of the circuit-board they are to be soldered in to. Those types of chips are unlikely to be built-in to a mid-price-range set of (Stereo) Bluetooth Headphones, that have an equalization function.

But what I can then speculate further is that some combination of alterations of these ideas should work.

For example, the convolution that is to be computed could be computed on the stream before it has been up-sampled, and it could then be up-sampled ‘cheaply’, using the linear interpolation. The way I had it before, the half-used virtual equalizer bands would also accomplish a kind of brick-wall filter, whereas, to perform the virtual equalizer function on the stream before up-sampling would make use of almost all the bands, and doing it that way would halve the amount of MIPS that a DSP chip needs to possess. Doing it that way would also halve the frequency linearly separating the bands, which would have created issues at the low end of the audible spectrum.

Alternatively, implementing a digital 9- or 10-band equalizer, with the
bands spaced an octave apart, could be achieved after up-sampling, instead of before up-sampling, but again, much more cheaply in terms of computational power required.

Dirk

## Comparing two Bose headphones, both of which use active technology.

In this posting I’m going to do something I rarely do, which is, something like a product review. I have purchased the following two headphones within the past few months:

The first set of headphones has an analog 3.5mm stereo input cable, which has a dual-purpose Mike / Headphone Jack, and comes either compatible with Samsung, or with Apple phones, while the second uses Bluetooth to connect to either brand of phone. I should add that the phone I use with either set of headphones is a Samsung Galaxy S9, which supports Bluetooth 5.

The first set of headphones requires a single, AAA alkaline battery to work properly. And this not only fuels its active noise cancelling, but also an equalizer chip that has become standard with many similar middle-price-range headphones. The second has a built-in rechargeable Lithium-Ion Battery, which is rumoured to be good for 10-15 hours of play-time, which I have not yet tested. Like the first, the second has an equalizer chip, but no active noise cancellation.

I think that right off the bat I should point out, that I don’t approve of this use of an equalizer chip, effectively, to compensate for the sound oddities of the internal voice-coils. I think that more properly, the voice-coils should be designed to deliver the best frequency response possible, by themselves. But the reality in the year 2019 is, that many headphones come with an internal equalizer chip instead.

What I’ve found is that the first set of headphones, while having excellent noise cancellation, has two main drawbacks:

• The jack into which the analog cable fits, is poorly designed, and can cause bad connections,
• The single, AAA battery can only deliver a voltage of 1.5V, and if the actual voltage is any lower, either because a Ni-MH battery was used in place of an alkaline cell, or, because the battery is just plain low, the low-voltage equalizer chip will no longer work fully, resulting in sound that reveals the deficiencies in the voice-coil.

The second set of headphones overcomes both these limitations, and I fully expect that its equalizer chips will have uniform behaviour, that my ears will be able to adjust to in the long term, even when I use them for hours or days. Also, I’d tend to say that the way the equalizer arrangement worked in the first set of headphones, was not complete in fulfilling its job, even when the battery was fully charged. Therefore, If I only had the money to buy one of the headphones, I’d choose the second set, which I just received today.

But, having said that, I should also add that I have two 12,000BTU air conditioners running in the Summer months, which really require the noise-cancellation of the first set of headphones, that the second set does not provide.

Also, I have an observation of why the EQ chip in the second set of headphones may work better than the similarly purposed chip in the first set…

(Updated 9/28/2019, 19h05 … )

## A New Set Of Headphones

As early as This posting, I had experimented with Bluetooth Headphones, that were specifically designed to handle High-Fidelity sound, for continuous music playback. But the fact is, that in the past 2 years, 3 such headphones failed me. The most-recent, ‘Infinim HBS-910′ set also failed on me, mechanically, only earlier this month, which effectively means that in total, if I continued doing things this way, I’d continue to burn through my money too fast.

So the course which I’ve chosen to go instead, is to use wired headphones, but to buy slightly-higher-quality, wired headphones, that are compatible with an Android device.

There once existed the observation, that the buttons on certain headphones would only work with iOS devices, and the buttons on other headphones would only work on Android-based devices. Interestingly enough, If the packaging doesn’t specify, then today, most headphones will work on either devices. My new Headrush HRB 3012 set has buttons which my Samsung Galaxy S6 recognizes.

(Edit 07/14/2018 : )

Their cord consists of a ribbon, instead of the older-type, standard elastic, round-cross-section cords, which I was used to. I think that the current, ribbon design is a clever way to minimize any injuries which a headphone-cord can sustain, let’s say because users often pull the headphones out of their socket, by the cord instead of by the jack. The only way I foresee the ribbon-design getting injured, would be if somebody got a knot into it – and was then foolish enough to try to undo the knot, by just pulling it tight. And, because the ribbon tends to be more stiff, undoing knots correctly, has actually become easier.

There is one little issue with these though. Like the designs that I was used to, this set of headphones has a bump in its ribbon, which splits into two ribbons: One to the left ear, and one to the right ear. And in the segment of ribbon to the right ear, there is a remote-controller-button, inline-mike bump. When all the ribbons are (untwisted) parallel, and at right-angles to the wearer, the ribbon that goes to the right ear, has its mike facing away from the wearer, and has the controller-buttons facing towards the wearer.

As a result, I find myself twisting or rotating the right-hand ear-phone 360⁰ at the end of its ribbon-segment, thereby turning the inline-bump 180⁰, so that the inline-mike is again, facing towards me.

Dirk

## LG Tone Infinim HBS-910 Bluetooth Headphones

In This earlier posting, I had written that my LG Tonepro HBS-750 Bluetooth Headphones had permanently failed. Today, I received the HBS-910 headphones that are meant to replace those. And as I’ve written before, it is important to me, to benefit from the high-quality sound, that both sets of headphones offer.

I’m breaking in the new ones, as I’m writing this.

There exists a design-philosophy today, according to which music-playback is supposed to boost the bass and attenuate the highest frequencies – the ones higher than 10kHz – so that the listener will get the subjective impression that the sound is ‘louder’, and so that the listener will reduce the actual signal-level, to preserve their hearing better than it was done a few decades ago.

1. The lowest-frequency (default) setting on the equalizer of the headphones does both of those things.
2. The next setting stops boosting the bass.
3. The third setting, stops attenuating the treble.

Overall, I get the impression that the highest frequencies which the HBS-910 can reproduce, extend higher, than what the HBS-750 was able to reproduce.