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Exteek C28 Bluetooth 5.0 Audio Transmitter/Receiver For only $10.99 (from eBay) or about $8 (AliExpress), this little device can act as either an audio transmitter or receiver. That means you can create a shortrange wireless audio link, convert a regular amplifier into a Bluetooth amplifier, use a CD player with Bluetooth headphones/earphones and more. AC7006F Features (taken from the data sheet) » CPU: 32-bit dual-issue DSP, up to 160MHz, single-precision FPU with CORDIC accelerator engine » SBC & AAC audio decoding supported » mSBC voice codec supports MP2, MP3, WMA, APE, FLAC, AAC, MP4, M4A, WAV, AIF and AIFC audio decoding » Packet Loss Concealment (PLC) for voice processing » Single/dual mic Environmental Noise Cancellation (ENC) » Multi-band DRC limiter » 20-band EQ configuration for voice effects » Stereo 24-bit DAC, SNR ≥ 102dB » Stereo 24-bit ADC, SNR ≥ 95dB » DAC sampling rates of 8kHz, 11.025kHz, 16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz, 48kHz, 64kHz, 88.2kHz & 96kHz » ADC Sampling rates of 8kHz, 11.025kHz, 16kHz, 22.05kHz, 24kHz, 32kHz, 44.1kHz & 48kHz » Stereo microphone amplifier with built-in bias generator » Can drive 16Ω & 32Ω speakers Bluetooth » Compliant with Bluetooth V5.3+BR+EDR+BLE specification » Meets class 2 and class 3 transmitting power requirements » Supports GFSK and DQPSK for all packet types » Provides a maximum +10dbm transmitting power » EDR receiver with -94dBm sensitivity » Fast AGC for enhanced dynamic range » Supports A2DP 1.3.2, AVCTP 1.4, AVDTP 1.3, AVRCP 1.6.2, HFP 1.8, SPP 1.2, SMP, ATT, GAP, GATT, RFCOMM 1.1, SDP core 5.3, L2CAP core 5.3, PNP 1.3 & HID 1.1.1 Review by Allan Linton-Smith siliconchip.com.au Australia's electronics magazine T he device has an internal battery and comes in a neat package with a 3.5mm to 3.5mm jack, a charging cable and a user manual. It can be charged from any 5V USB source and lasts for around four hours of use (140mAh). The receiver will power a small speaker or headphones as long as the impedance is at least 16W. You can make your average analog headphones or earbuds into a Bluetooth model by plugging them into this. It has many other practical applications. For example, if your TV has a regular headphone output jack, you could plug this in, set it to transmit mode and watch TV using wireless Bluetooth earphones. Or if you have a regular analog amplifier or receiver, plug this in, set it to receive mode and stream music from your computer, smartphone or tablet. You could also use it to convert an Aux input on an older car to Bluetooth, so you can stream music from your phone (an application suggested by the seller). Its signal-to-noise ratio and handling of low-level signals are good enough that you could even consider using it to turn musical instruments like electric guitars, or even microphones, from wired devices into wireless. The microphone would need to be battery-powered to become truly wireless. The heart of this device is a BPOY120356F4 chip manufactured by Zhuhai Jieli Technology Co Ltd (JL), who describes it as an “AC7006F Bluetooth Audio Chipset”. Performance We tested the performance using a pair of these as a Bluetooth wireless audio bridge. That means the performance reflects both the inbuilt ADC (analog-to-digital converter) of the transmitter and the receiver’s DAC (digital-to-analog converter). Therefore, our measurements can be considered a ‘worst-case’, and you will likely get better results using the transmitter or receiver alone. These tests also take into account the quality of the Bluetooth link itself, although most of our tests involved a sinewave signal, which would be a best-case scenario for a digital compression scheme. The transmitter unit was connected September 2024  29 Fig.1: this spectral (FFT) plot of the receiver output shows the 1kHz peak from the signal being transmitted plus the noise and distortion harmonics at all other frequencies, which are reasonably low. It is not quite CD quality, but it is close, with a signal-to-noise ratio of 93.6dB. For comparison, a CD has a maximum dynamic range of 96dB. to the audio generator output of an Audio Precision analyser and then paired with the receiver, which was connected to the analyser’s analog input. Pairing between the devices was automatic and took only a few seconds. With this configuration, we analysed the frequency response, distortion (THD+N) vs frequency, THD+N vs input level and inter-channel crosstalk. We also measured the signal-tonoise ratio. To eliminate the 192kHz carrier of the Class-D amplifier in the receiver, we used an AP AUX-0025 ‘brick wall’ filter together with an S-AES17 low-pass filter. These are necessary to enable accurate measurements because the slew rate of the analyser input stage (which uses AD797 op amps) is too low to handle the digital carrier. The manufacturer data sheet recommends a minimum load impedance of 16W, so we added a 32W dummy resistive load across the output for all measurements to make them realistic. Signal-to-noise ratio We determined the signal-to-noise ratio by spectral analysis (FFT) of the receiver output, delivering a 1kHz sinewave – see Fig.1. The noise spectrum is fairly low. The resulting signalto-noise ratio is 93.6dB, which falls short of perfection (we would prefer to get closer to 100dB), but it is suitable for most domestic uses. It is not far off from the best dynamic range you can get from a CD, which is around 96dB. Remember that this measurement includes the noise contributions of both the transmitter and the receiver. Frequency response Fig.2: the frequency response between the C28 transmitter and receiver was surprisingly good, having only a slight 2dB fall-off at the low end. The dip at the upper end (above 15kHz) is just an artefact of the brick wall filtering used to remove the residual high-frequency carrier. With the transmitter and receiver one metre apart, I injected a swept frequency signal into the transmitter at 500mV RMS. Because the signal is delayed slightly between being fed to the transmitter and coming out of the receiver, I had to delay analyser measurements by 500ms to ensure accuracy. Fig.2 shows the resulting frequency response plot. It is pretty good, with a slight roll-off below 100Hz, resulting in a response that is down by 2dB by 20Hz. The 0.5dB dip at the high end is just an artefact of the ‘brick wall’ filter. The result was better than expected, especially compared to older AV Australia's electronics magazine siliconchip.com.au 30 Silicon Chip transmitters/receivers we have tested previously. Total Harmonic Distortion The THD+N measurements gave fairly reasonable results of around 0.04-0.05% across the audio band. This is a combination of the distortion of the transmitter’s ADC, the receiver’s DAC, and the audio power amplifier in the receiver. Therefore, the distortion from either the transmitter or receiver would be lower. THD+N is usually measured with 80kHz bandwidth, but that is not possible with a brick wall filter as it will filter out any harmonics above 20kHz anyway. Given how flat the THD+N vs frequency plot (Fig.3) is, we don’t think it would make much difference. We also measured the THD+N against the input signal level (Fig.4), with the 1kHz signal fed to the transmitter swept from 1mV to 2V. It also turned out to be pretty flat at around 0.02-0.08% THD+N. We think this is due to the Class-D amplifier; it must introduce a non-linearity into the output signal that manifests as a floor on the distortion level. The receiver started to clip at around 900mV. Still, the distortion was reasonably good even at low input levels, again suggesting that this unit is suitable for microphone or musical instrument applications. It would be possible to use this with a record player (phono signal). However, you would be better off passing that signal through an RIAA preamplifier before feeding it to the transmitter. Fig.3: this plot of THD+N for the left and right channels with the signal frequency swept is the total distortion generated by the transmitter and receiver. This distortion level provides generally acceptable sound quality for domestic purposes. Availability We purchased our test units from eBay, via the link below, but the same ones appear to also be available from AliExpress at a slightly lower price: • eBay 133868488058 (siliconchip. au/link/abtn). • AliExpress 1005005459972095 (siliconchip.au/link/abto). Conclusion Compared to tests I have conducted on similar devices, this Bluetooth transmitter/receiver is excellent value for money, with reasonably low distortion and noise. It is not perfect, but you would need to pay significantly more to improve the quality even slightly. The inclusion of an internal battery and output amplifier makes it very versatile. SC siliconchip.com.au Fig.4: THD+N with a 1kHz signal of varying amplitude. The signal starts to clip at around 900mV, but the distortion is good at low input levels, making it suitable for microphone or guitar applications. Australia's electronics magazine September 2024  31