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Tangled up in Bluetooth (revisited)

  • Bluetooth in, analogue (or digital out). That’s the premise of iFi’s new ZEN Blue streaming DAC (£129) whose system-on-a-chip – a Qualcomm QCC5100 – routes the incoming data stream to your choice of 1) an ESS Sabre DAC followed by a discrete, fully-balanced analogue output stage (RCA or Pentaconn 4.4mm) or 2) a coaxial or TOSLINK output for connection to an outboard DAC.

    With design input from John Curl, iFi Technical Director Thorsten Loesch says that the ZEN Blue’s analogue stage “incorporates a range of high-quality circuit components, carefully selected for their performance in an audio context, including professional-grade balanced line drivers, C0G capacitors from TDK and a precision low-noise power supply IC from Texas Instruments.”

    In between the ZEN Blue’s input and output, we note support for the following advanced codecs that keep us away from the audibly weak SBC: AAC (hello iPhone), Sony’s LDAC (hello Android 8.0 and above), Huawei’s HWA and Qualcomm’s aptX HD (hello selected Androids).

    iFi’s press release goes further to tout the ZEN Blue as delivering “hi-res Bluetooth streaming to any audio system”. It is littered with references to “hi-res”.

    From the iFi press release: “The DAC stage handles sample rates well in excess of the maximum supported by the best hi-res Bluetooth formats – of the current 24-bit-capable codecs, aptX HD’s specification tops out at 48kHz, while LDAC and HWA support up to 96kHz.”

    The iFi PR team’s intent might be simply to illustrate the high-quality nature of their Bluetooth implementation but the preponderance of “hi-res audio” terminology might lead unsuspecting readers to believe the iFi unit is as capable of handling hi-res audio in the same manner as hardwired DACs do — converters that take their hi-res capability cues from the Japanese Audiophile Society and the USA’s Consumer Electronics Association who define hi-res audio support as (in essence): capable of losslessly handling PCM encoded at more than 16 bits and at a sample rate higher than 44.1kHz e.g. 24bit/48kHz.

    The ZEN Blue, like all aptX/LDAC-fuelled Bluetooth products, does not work that way. The missing ingredient is lossless handling which falls not to iFi but to Bluetooth itself. Lossless audio has yet to make it to the Bluetooth spec. Presented with a Redbook or hi-res audio stream, the ZEN Blue’s aptX HD and LDAC’s smart compression algorithms will move the audio from smartphone to Bluetooth receiver by discarding some data along the way. aptX HD can parse PCM streams up to 24bit/48kHz and Sony’s LDAC up to 24bit/96kHz but they can’t (yet) do so without some lossy compression.

    We might instead LDAC (and aptX HD) as a very good lossy codec whose upper limit of ∼990kbps is thrice that of MP3/AAC/Ogg’s 320kbps ceiling but insufficient for CD-quality (that maxes out at 1411kbps but typically streams at anything between 700kbps and 1100kbps), 24bit/48kHz PCM (that tops out at 2304kbps but on average streams at 1000kbps – 1250kbps) and 24bit/96kHz (which ranges from 2000kbps to 2500kbps and maxes out at 4608kbps). aptX HD comes up similarly short at a maximum data rate of around 580kbps. Qualcomm’s claim of ‘Near CD-quality’ should have us asking “Just how close exactly?”.

    Could this be why Qualcomm markets their aptX HD codec as “High Definition” and not “hi-res”? Note the quotation marks (my bold emphasis) that surround their strapline: better-than-CD listening experience. A tactic that echoes Amazon Music’s newly announced HD tier that reframes CD-quality streaming as “HD”.

    Last week, Bowers and Wilkins announced a successor to their popular PX over-ear Bluetooth headphone. Each of the new PX7’s (US$399) earcups carries a 43mm dynamic driver, juiced by internal amplification that in turn feeds on a DAC sucking on (optional) DSP-powered, 4-microphone active noise cancellation (ANC). Battery runtime is rated at 30 hours between charges with a 15-minute recharge netting five hours. Impressive.

    At the front of the PX7’s playback chain sits Bluetooth whose advanced codec support options AAC, aptX, aptX HD and aptX Adaptive. The varies the amount of discarded data according to the strength of the Bluetooth connection and its available bandwidth to ensure a drop-out free listening experience. In other words, it relies on lossy compression to do its job. Per the iFi DAC, Qualcomm’s aptX HD compression can parse 24bit/48kHz files. It just cannot do so losslessly.

    Side note: only by hardwiring its USB-C socket to a PC or Mac can we pull lossless audio into the PX7; a very neat feature for deskbound listeners.

    None of this means Bluetooth streaming can’t sound very good. It absolutely can, especially when it pulls ANC into the picture. One of this commentator’s favourite products of all time is Sony’s 1000X series of over-ear headphones and I will purchase the B&W PX7 when they go on sale.

    However, the upshot of any linguistic slip-n-slide with hi-res support is confusion in the marketplace. Confusion that could cause consumers like you and audio tech writers like me to conflate “HD” and “hi-res” and to think we are getting something that we are not.

    At least, not until the Bluetooth Special Interest Group writes support for the lossless transmission of digital audio into the A2DP Bluetooth specification. In the meantime, AV Hub’s LDAC Q&A on Sony’s ‘hi-res creep’ remains essential reading.

    Further information: iFi | Bowers & Wilkins

    You can read the first Tangled up in Bluetooth article here.

    Written by John

    John currently lives in Berlin where he creates videos and podcasts for Darko.Audio. He has previously contributed to 6moons, TONEAudio, AudioStream and Stereophile.

    Darko.Audio is a member of EISA.

    Follow John on YouTube or Instagram

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