Sometimes you would like to go wireless for audio but you still love your cabled stuff. Maybe it’s a pair of quality amplified speakers, or maybe it’s a set of cabled noise cancelling headphones. Whatever the reason is, you’re probably aware that buying products that were designed to be wireless in the first place is often an irreversible quality compromise.
As a result, I went looking for a Bluetooth A2DP receiver that would be self-powered and provide a jack for a 3.5mm device to plug into. Then, any existing A2DP compliant Bluetooth source device can be used to drive it.
That resulted in me ferreting out this product – a cheap product from China.
It pretty much should do what I wanted, or so I thought.
It claims that you can directly connect it to a speaker with 3.5mm audio cable, and that it would be called I-WAVE.
Inside, is a small user manual, the unit itself and a USB cable for charging purposes. It can also be used to power the receiver while it is in use – a handy addition if the Hi-Fi system you are connecting it to has a powered USB port you can “steal power” from.
The bottom edge of the device contains the 3.5mm jack and the mini USB-B connector.
The left side contains the hardware slide power switch, and the top has the red and blue status lights which indicate pairing status. The device needs about 15 seconds after powering on to go into pairing mode, where it alternately flashes red and blue. Once connected, it only blinks blue. If the connected device disconnects, the unit returns to pairing mode automatically. The Bluetooth name is actually DL-LINK
Of interest was the chipset employed in the product. Specific Cambridge Silicon Radio (CSR) chipsets support audio quality enhancement through the use of an alternate codec known as apt-X. In order to find out, I had to pry it open. No screws were used in the construction, but the two halves did fit quite tightly together.
We can see it is based on an Omnivision OVC3860 chipset, which is a low cost chipset supporting only the regular SBC codec using Bluetooth v2.0 with EDR. There appears to be a chip marked PT2308-S which is a headphone amplifier opamp, and another chip ahove the OVC3860 which is probably an EEPROM containing configuration data.
The antenna is made from a printed trace on the top of the device, unfortunately, it doesn’t seem to be highly sensitive with a good range of only about 3-4 meters before audio interruptions start becoming frequent.
The whole unit is actually a DL-LINK TS-BT35A01 Ver 1.1 and is powered by a BYT 502030 Li-Polymer cell with 250mAh capacity.
The device was found and paired without problems using CSR Harmony.
It appears to support secure simple pairing, so no keying of any passkey was required.
A connection was successfully established to the device by manually browsing to it from within My Bluetooth Places. It seems to support AVRCP and HSP, but the device itself doesn’t offer any mic input or control buttons, so it’s likely because it’s running in a default configuration.
Seeing as I had the PicoScope, I decided to use it to quickly check out the performance of the device. Keeping in mind the PicoScope was operated without resolution enhancement, its dynamic range is only 8-bits which should be a lot less than the device. Instead, we can look at gross figures like frequency response.
Playing the output into the oscilloscope only, as a high impedance load, it’s got a very good frequency response across the board. You should only look at the top of the envelope – the jaggedness is because I did a quick two minute frequency sweep played back from the computer.
There’s a little loss of response at the DC end, but that’s normal. The frequency anti-aliasing filter isn’t that aggressive, with a little aliasing at the top end. Seeing that the device had an op-amp stage driving the output, I wanted to see what it was like driving a set of 32 ohm headphones …
As you can see, there’s a very definite loss of bass and the audio sounds very tinny. This is exactly why many “low cost” devices should not drive headphones directly, and instead will need a headphone amp to sound “proper” – the headphones just so happen to form a filter which blocks the low frequencies.
So, you can use this receiver for amplified speakers – just don’t use it for headphones.
Trying to look for intermodulation distortion (IMD) using the PicoScope in 8-bits isn’t going to be a very fruitful exercise. You just can’t spot any – maybe I should have tried using it with the full resolution enhancement available, but it’s just not a good idea to try judging a 16-bit DAC with something less than 16-bits.
I couldn’t leave this post without showing what some other things look like, for example, my USB connected Creative Soundblaster Digital Music LX on a much slower sweep – visible is a slight passband ripple and a more aggressive anti-aliasing filter that does impinge on frequencies above 20khz.
Looking for IMD with this one seems to show up quite strong spikes – so really, even the cheap BT audio receiver might be better …
Just for laughs, this one was recorded from my Realtek on-board sound card, that has plenty of IMD.
It might not have a brand, but it’s not too bad if you intend to use it with amplified speakers. The range of the antenna is a little short, which proves to be a slight annoyance, but the audio quality seems to be sufficient especially considering what the Bluetooth A2DP SBC compression will do to it.