Phonak TVLink-an analysis of its performance wrt bluetooth delay

The Phonak TVLink (and its predecessor Voiis) - Bluetooth delays.
Much has been said by Phonak, and users, about Bluetooth audio delays and the effects upon the user. Phonak promote the TVLink as having “TV to hearing system in the blink of an eye – an amazingly short transmission delay ensures listening pleasure even for open fittings.” and “The unique StereoSound guarantees a perfect listening experience.”
So is it??
The Voiis fails the lip sync test and is not useful for listening to TV sound.
The TVLink passes the lip sync test but may not be useful to a user who watches TV with the TV speaker in use (for a non-hearing aid user to listen) and who does not have at least a 50db loss across the whole audio spectrum.

There are two main issues with audio delay (for the purpose of this dialogue only that introduced introduced by the Bluetooth link to the Icom is considered). Firstly lip sync (mouth movements out of step with voice) and secondly audio distortion such as echo or timbre effects brought about by the interaction between the direct sound and the Bluetooth delayed sound.
To analyse these it is necessary to identify the delays produced by the TVLink, and Voiis and then to discover how susceptible we are to these effects.
I set out to measure these delays and to research acceptability.
Measurements of delays.
This turned out to be much easier than I anticipated.
Firstly a source. The UK BBC transmits nightly on their Freesat HD channel a 2 minute test sequence every two hours during “out of programme times”. This includes a lip sync test sequence, but the only part I needed was the sound element of this – a sharp sound that repeats precisely every one second. I recorded this.
Secondly to set up to measure the delay. I installed “Audacity”, free sound program from on my laptop. I plugged a separate microphone into the laptop and then put this on a padded surface with one of my Phonak Audeo Smart’s speakers right up against the microphone and my Icom draped over it. I turned the microphone gain upto near maximum (in Audacity) and then replayed the recorded BBC lip sync test. The microphone was thus able to pick up the sound from the hearing aid and the sound from the TV speaker. I set Audacity to record, firstly with the TV speaker on mute, I let it record for 10 seconds or so, then I unmuted the TV speaker and recorded a few more of the one second sounds.
I examined the sound traces in Audacity and it was very clear where the sound started each second. I expanded the scale of the display until I could see easily the 0.01 second interval markers. All I had to do then was to compare the start time with and without the TV speaker muted to see how much earlier the sound started with the TV speaker in operation.
The results? Well for the TVLink the difference is 40milliseconds and for the Voiis 110 milliseconds. I estimate that these are correct within 5 milliseconds. I also tried with the Icom fed via its wired input – in this case I could detect no difference, again an error margin of 5 milliseconds.

Acceptability – Distortion due to direct/delayed sound combining
“Acceptable Processing Delay in Digital Hearing Aids
For people who wear hearing aids, sounds are transmitted into the ear canal via two different paths. In the first path, sound travels around the hearing aid or past the earmold plumbing directly into the ear canal. The more “open” the fitting and the more normal the hearing, the greater the contribution of this pathway. In the second path, the signal passes through the hearing aid, is processed, and is then delivered into the ear canal.
These two paths have inherently different time delays before a given sound is perceived by the hearing aid user. Direct sound heard via the first path has the least delay, and is perceived before the amplified sound, which has to be first processed by the hearing aid circuitry.
While the delay caused by digital hearing aid amplification is relatively short, and measured only in milliseconds, the sound quality of the speech signal may still be compromised. We know that, when the delay is about 10 milliseconds, the direct sound and the amplified sound interact with each other and cause spectral ripples, which alter the timbre of the sound. Because these spectral ripples are only present when the two sounds are of a similar intensity, they are not present when the amplified sound is significantly louder than the direct sound. When the delay becomes longer, the patient may notice an “echo effect,” and when longer yet (more than 40 ms), the auditory information may fall out of synchronization with the visual information and interfere with speech reading.
For normal-hearing listeners, even speech production is affected when the delay exceeds 30 ms. Another group of researchers working with actual commercial hearing aids, however, found that delays as small as 10 ms affect sound quality.6
A factor that makes hearing aid delays even more of a concern in recent years is the popularity of open-canal (OC) fittings. As the term suggests, OC fittings leave the ear canal much more open than traditional earmolds or custom instruments. As a result, more sound travels directly into the open ear canal and is potentially more audible to the hearing aid user. Moreover, patients who typically wear OC fittings are those with only a mild-to-moderate hearing loss—the same group of users who have the greatest potential to be affected by hearing aid delay.
A recent study showed that, in OC fittings for simulated hearing losses that started at 2 and 1 kHz, processing delays may need to be as low as 5 and 6 ms, respectively, to avoid compromises in sound quality.”

To discover a relationship between the delays and the relative levels of the two sources (the direct one and the one via the hearing aid) I looked to the ITU (International Telecommunications Union). An extract from their Recommendation G.131 may be found here:
This, in Figure 6, identifies that to be “acceptable” the difference between the two sources should be more than 30db for a 40ms time difference.( I picked the 40ms time point to quote for reasons above)
So it would appear that as well as the delay we need to take into account the relative levels of the two audio paths. The latter will be very dependent upon the individual – profile of hearing loss and fit/venting of the hearing aid. An open fitting (as against a moulded fitting) is obviously more susceptible as is a non-flat hearing loss (with a typical high frequency loss the ear will receive through the vent sufficient low frequencies to be audible with no delay, but the high frequencies will only be heard via the delayed Bluetooth path)
Having discovered that the TVLink has a delay of 40 ms and that as a result there should be at least (from UTU G.131) a 40db difference in the levels of the two signals, this at least quantifies the vent path and hearing loss profile somewhat.
I do not know what attenuation there is though a moulded, or open, fitting, but say it is 10db then crudely this would imply that the maximum deviation in hearing loss across the frequencies could be 50 db (40db from ITU plus the 10db through the vent).
For vent effect see This suggests, Table 9, that the vent source sound can change the sound at the eardrum by 10db at 2kHz – and it can be increased or decreased!
In my case I have a loss of circa 20db at 250Hz and 80db at 3khz – a difference of 60db. I have tried the TVLink ( in fact two of them in case the first was faulty) and its unusable. The sound is extremely hollow and speech is difficult to understand.
Acceptability - Lip Sync
Accepted tolerances for lip sync (video to audio synchronisation) are that the sound program should never lead the video program by more than 15 milliseconds, and should never lag the video program by more than 45 milliseconds. So the Voiis fails on this count (which was expected) and the TVLink passes.

The Voiis fails the lip sync test and is not useful for listening to TV sound.
The TVLink passes the lip sync test but may not be useful to a user who watches TV with the TV speaker in use (for a non-hearing aid user to listen) and who does not have at least a 50db loss across the whole audio spectrum.
(Am I correct in this 50db deduction??)

Ooops, I messed that up!!
I “moved on” from the 30db from the ITU figures and transposed this to 40db in error.

So where I said 40db should read 30db and where I concluded that a 50db marging was required this should be 40db

What on earth has your “response” contributed?
I suppose at least you did not advertise your business in New Delhi unlike your “contribution” to another thread.

Very thorough explanation and data. You have taught me something. I was basing my explanation of the delay on what I have been told by Phonak and not by independant research like you have conducted.
Not that it would be greatly significant but how do you rule out any delay from the sound travelling to the mic and reaching the software (there must be some processing time I would think)?

I can see how plugging the ICOM in directly would indicate the difference for the most part.

Thank You for taking the time to do this and post the results.


Thank you for taking the time to read the post.
Mot sure what you ask in:
"how do you rule out any delay from the sound travelling to the mic and reaching the software "

There clearly is a delay for the audio from the TV loudspeaker to my ears/ the microphone I used in my measurements. I placed the microphone approx where I sit - some 2.5 metres away from the TV.
(I did incidently calculate how far away I would have to be from the TV speaker for the air borne audio delay to equal the TVLink bluetooth delay - the answer is 44 feet; unfortunatly my lounge is not that big!! - see this post for details - )

This is the only delay which is outside my measurements for all delays in the PC processing would affect the two audio sources (air borne from the TV speaker and via bluetooth Icom or Voiis) exactly the same. It would make no difference to the result if the PC took say a minute to process the sound it was picking up.

Hope this makes sense to you.

Hi Geeko

It’s a very fine report you have made. And the conclusion it also right the only part I didn’t like is the assumptions you make about where in the system there is a delay.

First the TV, I assume that the voiis/tv link connected with a wire to the tv. Have you made sure that the output to the wire and the speaker on the TV are in sync. This is NOT the case on all TVs.

Second the HA also has a delay, and depending on the settings in it this this will also vary. Not knowing what is exactly is done in the HA I can’t tell what value it’s.

but all in all these few things are not going to changed picture, with a blue tooth based system the delay can be a problem for some.:wink:


There are of course delays all the way through the system, I accept that.
The way I measured the delays eliminated delays in the PC and its software (as the same channel was used for both the air borne sound and the Icom fed sound).
There are of course delays through the hearing aid, however when I measured the delay using the wired input of the Icom, I could find no delay, but as I stated there is an uncertainty of 5msecs in my measurements due to the rise time of the audio signals I was using.

It will be the case that the line output from the TV is not in precise sync with the speaker output, but as I know there is little more than a class D amplifier in the loudspeaker path then any delay will be very mush less than 1msecond.
There is of course a further complication in that many TV programmes do not have a correct lip sync relationship, but this is not an issue for me as I was only really concerned with the audio distortion which I found occurred.

Interestingly there is a good paper on the delays wich hearing aids themselves introduce, due largely to the band filtering which aids introduce and that aids with more channels are worse in this respect see -

Hi Gekko

The main reason why I mention a delay in the audio path in the TV, is that some hear impaired have reported back that the unacceptable long delay when using there system with the TV but not then listing to the radio. I made a small investigation and discovered that on some TV’s there is a delay between the line-out and speaker on more then 100ms. Where the speaker output was in sync with the picture but the line-out wasn’t. But generally your conclusion is right that a bluetooth system will add a delay that can be disturbing to some.

100ms is an amazing delay within a TV between the line output and the speaker, I cannot imagine how that is happening; something inside the TV has to be storing the sound for 100ms.

My first check was the delay between hearing aid output fed from the TV line output and the audio from the speaker, whyere I found less than 5ms, so it would seem that my TV (Panasonic LCD) does not have that built in delay.
You can see from my opening post that 45ms is considered the max accep0table delay before lip sync becomes a problem so why any TV would have 100ms is a mystery.
Do you know the make and model of the TV with that delay? I would like to try and find out whats going on

The problem as I remember was with digital tv broadcast and picture processing time vs sound processing. On some TVs the sound to the speaker was delayed so it was in sync with the picture but the audio to the connectors wasn’t. I expect this was so a surround sound amp could delay it, accordantly to its own processing delay:)