What is aptX Bluetooth Codec?

How Does the aptX Bluetooth codec work? And Is It Really Any Better?

aptX_logo.jpg
CSR

Audio enthusiasts know aptX as a technology that purportedly improves the sound of wireless speakers and headphones. According to CSR, the semiconductor company that licenses aptX, the technology is in more than 500 different products and 600 million individual devices, and has been licensed by more than 280 companies. Surprisingly, though, the industry generally accepts aptX’s quality claims without question.

The recent introduction of aptX into Panasonic TVs got me thinking it was time to dig deeper into the topic. So I requested an interview with Jonny McClintock, director of sales and marketing for aptX. McClintock was surprisingly open and frank with his answers, and didn’t hesitate to go into great depth about the technology.

I also put together an online aptX vs. SBC blind listening test you can do to hear the difference.

AptX is a codec, or code/decode algorithm, that packs a digital audio signal into a smaller amount of data transmission bandwidth or data storage than the signal would otherwise occupy. It’s a lossy (as opposed to lossless) codec, which means that it does not completely reconstruct the original audio signal. Instead, a lossy codec chooses to discard the audio data that should have the least impact on fidelity.

Many audio enthusiasts are under the impression that a certain codec is better or worse than another, but it's more accurate to say that different codecs are optimized for different applications.

Some may be fine-tuned for squeezing audio signals into a very low-bandwidth Internet stream, while others may be designed to make best use of the ample storage space on a Blu-ray disc. Some are designed for applications where plenty of computer power and memory is available, while others are designed for devices with limited computing power.

The core technology of aptX dates back to the late 1980s, around the time MP3 was born. An early version of aptX was used for the theatrical version of DTS (but not the home version). McClintock told me it’s used for many audio-over-IP (Internet Protocol) applications, and it’s currently in use in about 30,000 radio stations worldwide. The version of aptX used in Bluetooth was introduced in 2008.

Advantages of aptX

McClintock told me that aptX has two advantages over SBC (Sub Band Codec), which is the standard codec used with Bluetooth. First, of course, is better sound quality. Second is low latency, or the time it takes for the system to encode, transmit and decode an audio signal.

“SBC is a variable bit pool codec, and when Bluetooth came out it was typically configured for the lowest possible bitrate, which is under 200 kilobits per second,” McClintock said. “It doesn’t sound good. It rolls off above 16 kilohertz, the upper end [treble] sounds grainy, and the signal-to-noise and harmonic distortion performance is poor.

“AptX runs at 354 kilobits per second, and it’s a fixed 4:1 compression ratio so the performance is guaranteed,” he continued. “I’m not saying one codec is better than the other; SBC and MP3 and AAC have more aggressive compression and are thus far more efficient than aptX.

But if you have the capability of addressing more bits it’s going to sound better, just like DTS versus Dolby Digital.”

While SBC can operate at bitrates up to 345 kilobits per second, according to McClintock manufacturers don’t implement it that way.

Late Arrivals

Latency is also a potential problem with SBC, AAC and MP3. McClintock said that  latency with the low-latency version of aptX is 40 milliseconds in comparison to 220 ms +/-50 ms for SBC and 800 ms +/-150 ms for AAC at 256 kilobits per second. I can’t confirm the aptX and AAC latency figures he cites, but I have noticed that measurements I’ve done of devices using SBC do indicate his latency figures for that codec are correct.

Of course, 200 ms (1/5th second) of latency doesn’t matter if you’re playing music through headphones or speakers, and probably not even if you’re making a phone call.  McClintock said the advantage comes when you’re watching video or gaming. “If you want to play the sound from a TV through your Bluetooth headphones, and have the actors’ lips sync with the audio, you need to get the latency down to 40 ms because that corresponds to the frame rate [which is 33.3 ms per frame for 30-frame-per-second video]. With other codecs you’re going to end up with something that looks like a spaghetti western.”

McClintock said the reason why the latency of aptX is so much lower than with other codecs is because they are frame-based, whereas aptX is sample-based. With a frame-based codec, an entire data frame (containing the core audio samples plus the information about how to unpack them) must be received and loaded into a memory buffer before it can be decoded. In comparison, while aptX does use data frames, the decoder can begin its work as soon as it starts receiving samples, rather than waiting for the rest of the data in the frame to arrive.

It seems to me that TV and gaming may be the best application of aptX in Bluetooth. The latency advantage of aptX should be easily noticeable in these applications. As we found in a recent test on The Wirecutter, most wireless headphones made for TV listening are cheap, horrible-sounding junk. Building aptX Bluetooth into a TV would let you use whatever aptX Bluetooth headphones you already have and thus save some money while probably getting much better sound.

The Core Tech of aptX

AptX differs from most audio codecs in that it relies primarily on ADPCM, or adaptive differential pulse code modulation. The “adaptive differential” part of it means that instead of transmitting or storing complete audio samples, aptX starts with an audio signal, predicts what the signal will be next based on what it was before, then transmits only the difference between the original sample and the predicted sample.

Most of the codecs typically used for consumer audio, including MP3, AAC and Dolby Digital, take advantage of the fact that a loud sound at a certain frequency will tend to mask quieter sounds in the same frequency band. These codecs will eliminate the quieter sound on the theory that its omission will be inaudible, by simply removing samples that the coding algorithm deems unnecessary. “The signal with the lower amplitude is removed and never reappears,” McClintock said, “and that’s why you get the general removal of the final details of audio.”

The Case of the Missing Brand

I bought my Samsung Galaxy S III smartphone in large part because I was told by a PR person at CSR that it has aptX encoding. Yet I was disappointed to find no mention of aptX anywhere in the phone’s interface; no aptX logo anywhere on or in the phone; and no way to turn aptX on and off. For all intents and purposes, I have to take CSR’s word that aptX is in there. From what I’ve seen, this is the case with most tablets and smartphones that are listed as aptX-equipped. Why? I asked McClintock.

“It’s a constant challenge for our team,” he replied. “All of the sink devices [speakers, headphones, car stereos and other devices that receive a Bluetooth stereo audio signal] have a way of showing aptX is inside. But we still have some way to go with the big phone and tablet companies into making sure the logo is in there."

Fortunately, the aptX website now has a pretty extensive and up-to-date list of the products that use aptX.

AptX is now in about 70 percent of Android phones and tablets, but it’s not in iOS devices such as iPhones and iPads. Why not? “I suspect Apple probably has a dream to keep audio in the AAC format [used for iTunes] all the way from the cloud to the phone to the headset, although that would present challenges because they'd also have to fold things like ring tones into the AAC stream,” McClintock replied. “Also, Steve Jobs apparently didn’t like to use technologies he couldn’t own, and aptX is owned by CSR. I’d like to think it has to do with a philosophical issue, not with the performance.”

The Bottom Line

Of course, all this technical talk might fascinate audio enthusiasts, but most of the people who buy Bluetooth speakers, headphones and car audio systems don’t care about all this stuff. So I asked McClintock if he could finish our interview with a simple “elevator pitch” that anyone could understand.

“The top end [treble] has a lot more clarity, especially in the transients, which SBC can’t deal with,” he replied. “And the stereo imaging is much better with aptX.”

Want to find out for yourself what the difference is? Take my online aptX vs. SBC blind listening test.