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Name: Benoit Nadeau
Member since: 2002-04-03 17:23:09
Last Login: 2014-04-22 17:48:00

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Homepage: http://benad.me/

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I am Benoit Nadeau, jr. eng. in Software Engineering,
living and working in Montreal, Canada.

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Good Enough Wireless Audio

For over six months there have been strong rumours that Apple will drop the 3.5mm headphone jack in the next iPhone, something that may have a large impact on the market of portable headphones. Either they will have to adopt the proprietary Lightning connector, or use the more standard Bluetooth wireless protocol.

This isn’t too surprising. I’ve noticed that with the latest Apple TV, the entirety of the Apple product line supports Bluetooth headphones, as if to prepare the market for a more “wireless” headphone future.

Still, if that were to happen, it would suck. And I’m not the only one to preemptively complain. I’ve avoided wireless headphones since they used to greatly reduce the sound quality, on top of the inconveniences of limited range and batteries that need recharge.

Unrelated to this Apple rumour, I did try out wireless headphones (of many kinds). So, are wireless headphones in 2016 good enough?

My first pair of wireless headphones is the Sennheiser RS 175. It is made for home use, as it requires a wireless base that also acts as a charging station for the headphones. It is using some kind of digital connection (up to 96 kHz PCM at source, and using 8-FSK digital lossless), so you either get the full quality or none at all, unlike analogue signals that would degrade the source quality depending on the interference. For me, the primary use for those headphones was to use them at home with some freedom of movement that wired headphones wouldn’t permit, and also to not have wires on the middle of the living room anymore. I was pleasantly surprised at the sound quality and how long the battery charge lasts.

Yet, those avoided my main concerns about Bluetooth headphones: Is the sound quality allowed by the Bluetooth protocol good enough? Some background about Bluetooth audio protocols first.

The first kind of audio supported by Bluetooth was for phone calls, so the audio quality targets what is typically needed for phone lines and not much more. This was done through the Headset Profile (HSP) for headsets, and the Hands-Free Profile (HFP) for cars. Both support 64 kbit/s signals, be it uncompressed, µ-law or a-law, commonly used in telephony, or CVSDM. For music, this is pretty bad, and I suspect my early experiences with music over Bluetooth was through those profiles.

Later, Bluetooth supported “proper” music streaming though the Advanced Audio Distribution Profile (A2DP). While it can support modern MPEG audio codecs (MP3, AAC, etc.), the only required codec is SBC, the only one that was available for free for use in Bluetooth applications. There’s also the newer and better aptX, but most devices don’t support it, notably all iPhones, maybe due to licensing costs and patent protections. And since MPEG codecs are even more expensive to license, it means the only codec commonly supported is SBC. And how good is SBC? Well, good enough compared to other codecs at that bit rate. In plain English, you can hear some quality loss if you listen hard enough. The quality loss is comparable to 256 kbit/s MP3s, which is fine but not great.

So, before jumping into Bluetooth headphones, I tested out Bluetooth audio with two different portable Bluetooth receivers. The first is the OT-ADAPT, made for outdoor sporting, where you would place your phone in your backpack and use a standard set of headphones connected to the receiver outside. The second is the MPOW, this time made for car stereos, but portable enough to be used outdoors. What I noticed with both is that the audio quality is far more impacted by the quality of the DAC than the SBC codec in the first place. The MPOW is generally louder than the OT-ADAPT (since it typically targets “audio line in” at full volume), but even at comparable volume it has far fewer background noise in the output signal. Still, with either receivers, the sound quality loss cannot be noticed with any sub-$100 headphones at average volume.

And then I finally made the jump to Bluetooth-enabled headphones, with the Sennheiser Momentum M2 AEBT. Testing with the provided cables with wired “airplane mode” use, I noticed no quality difference between wired and Bluetooth wireless audio, even while using SBC with my iPhone (the M2 does support aptX). The price difference for the premium of having Bluetooth is difficult to justify compared to sub-$30 Bluetooth receivers, but then those headphones have amazing sound quality and reasonable active noise cancellation.

So, what’s my recommendation? If you simply want some freedom of movement or Bluetooth-enable your car’s audio system, the MPOW Bluetooth receiver should be good enough. Otherwise, you may want to wait for a few months, as the release of the next headphone jack-free iPhone may spur a new wave of Bluetooth headphones, driving down the price of older models. And don’t worry too much about sound quality: It’s good enough.

Syndicated 2016-07-07 22:44:14 from Benad's Blog

Flattening Circular Buffers

A few weeks ago I discovered TPCircularBuffer, a circular buffer implementation for Darwin operating system implementations, including Mac OS X and iOS. Now, I’ve implemented circular buffers before, so I though there wasn’t much need for yet another circular buffer implementation (let alone one specific to iOS), until I noticed something very interesting in the code.

A trick TPCircularBuffer uses is to map two adjacent memory blocks to the same buffer. The buffer holds the actual data, and the virtual memory manager ensures that both maps contain the exact same data, since effectively both virtual memory blocks remaps to the same memory. This makes things a lot easier than my naive implementations: Rather than dealing with convoluted pointer arithmetics each time the producer or consumer reads or writes a sequence of values that cross the end of the buffer, a simple linear read or write works. In fact, the pointers from that doubly-mapped memory can be safely given to any normal function that accepts a pointer, removing the need to make memory copies before each use of the buffer by an external function.

In fact, this optimization is so common that a previous version of the Wikipedia page for circular buffers had some sample code using common POSIX functions. There’s even a 10-year-old VRB - Virtual Ring Buffer library for Linux systems. As for Windows, I’ve yet to seen some good sample code, but you can do the equivalent with CreateFileMapping and MapViewOfFile.

Both Wikipedia’s and VRB’s implementations can be misleading, and not very portable though. On Darwin, and I suspect BSD and many other systems, the mapped memory must be fully aligned to the size of a memory page (”allocation granularity” in Windows terms). On POSIX, that means using the value of sysconf(_SC_PAGESIZE). Since most of the times the page size is a power of 2, that could explain the otherwise strange buffer->count_bytes = 1UL << order from Wikipedia’s sample code.

By the way, I’d like to reiterate how poor the built-in Mac OS X documentation is for POSIX and UNIX-like functions. Though it does warn pretty well about page size alignment and the risks involved with MAP_FIXED of mmap, the rest of the documentation fails to mention how to set permissions of the memory map. Thankfully, the latest Linux man pages for the same functions are far better documented.

Syndicated 2016-05-23 18:01:52 from Benad's Blog

The Static Blog

A quick note to mention that I added The Static Blog to my main web site, discussing the relocation of the blog you’re reading right now to this site.

Syndicated 2016-04-27 01:11:57 from Benad's Blog

Final Blog Move?

This is a short post to mention that my blog, originally hosted on Squarespace as blog.benad.me has now moved alongside my main web site under benad.me/blog.

All links to each post and the RSS feed should now automatically redirect to the new location. This may have created some duplicates in your feed reader.

There is still some clean up to do to make the older posts look better, and I need to post a longer article to explain the rationale behind this, but for now everything should be working fine. The blog.benad.me domain will remain active as an option if something were to happen, but for the time being my blog should remain where it is.

Syndicated 2016-03-29 19:51:33 from Benad's Blog

Amazon Cloud Drive Backend for Duplicity

To follow up on my previous post, I wrote acdbackend, to add Amazon Cloud Drive support to duplicity, by wrapping around the acd_cli tool.

Actually, adding online storage services to duplicity is pleasantly easy to implement. If you have some command-line tool for your service that supports download and upload using stdin/stdout, file listing, and deleting files, then in an hour you can have it working in duplicity.

Syndicated 2016-03-03 02:16:19 from Benad's Blog

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