|Ideal response curve|
Number two was connectivity. With only a few exceptions noted below, once the connection was established the Bluetooth STL was quite reliable.
|Real world response curve|
Some of the units tested provided internal and cell phone battery saving features that put the Bluetooth unit in sleep mode after a short absence of audio signal.
Worse, some of these provided signal tones to alert the user to the sleep and wake states of the Bluetooth.
When in the audio chain, these tones came through at an ear-splitting level and the noises emanated by the transmitter sounded like bim - pew - thunk every time the chip went in and out of sleep mode.
Our transmitters tend to have a fairly narrow window, with FM transmitters usually in the area of 100 Hz to 10,000 Hz and AM transmitters an abysmal 500 Hz to 8,000 Hz. FM transmitters usually offer a pre-emphasis circuit to help the top end and a compressor can help the bottom in both AM and FM.
Audio chain components are usually compatible with the AM or FM audio bandwidth but a Bluetooth STL ouch occurs when the audio chip adds another curve to the picture.
One of the units tested had so much bottom that the result was impossibly muddy, another was too strong in midrange and a third was hopelessly shrill.
The third and perhaps most frustrating ouch factor was the lack of transparency in specs and chipsets. Our faithful readers might be wondering why no brands or models are mentioned in this report. The reason for that is that testing showed that brands and models are not dependable predictors of the chipsets used and therefore not predictive of success or failure.
Bluetooth audio devices are manufactured in batches using inexpensive chipsets built by offshore factories and the same make and model, purchased at different times, will have different chipsets. There seems to be no way to make a dependable recommendation.
Instead, the recommendation is to purchase your Bluetooth STL components from a vendor with a liberal return policy. If you get a unit with an intolerable bim-pew-thunk or a a set with a response curve that's a poor match for your transmitter audio, send it back and try another.