What causes wobbly sound on audio cassettes?

Audio cassettes generally last a long time. (we have many in our library that are more than 30 years old) however we're starting to get a few come in for digitsing that are exhibiting problems. Before you throw them away - let us see what we can do with them. Usually the tapes will play ok in our professional cassette decks but every now and then we'll have to explore the problems more deeply to get a good quality transfer. 

Ampex audio cassette

We recently received a tape of an oral history project that was recorded in 1985. During the assessment process we found that it would play for around 3 minutes and then start to play back erraticly, eventually slowling to a stop. You can hear the effect here and it might be something you've heard before and then thrown the tape away. Sometimes people think the tape has "stretched" in fact tape is very robust and that's a rare occurence.

There are a couple of possible reasons for this playback problem and with some careful work we've been able to retrieve the audio. With this particular tape it  was a one-off recording, no other copies exist so it was of great importance and we had to do our best to get a good copy. 

Could this cassette tape be exhibiting a problem we usually find only with reel to reel tapes called "sticky tape syndrome"? We've only just started to find cassettes with this problem but it was certainly old enough to be a candidate.

We have a special temperature controlled oven for "baking" audio tapes to stabilise the tape surface. (Warning: Please do not attempt to do this yourself as you may ruin the tape permanently) When we've gone through this process it gives us enough time to do the digital transfer of the tapes. In this case a few hours in the oven stabilised the tape surface enough for us to be able to make a copy and then go through our audio restoration processes to improve the audio. 

You can hear the result of the new transfer here.

The client was fortunate that this tape was caught in time before it became completely unplayable and we've been able to retrieve the important interview contained on it.

If you have a library of tapes that are approaching 15-20 years old it would be a good idea to have them checked and transferred to another format as soon as possible,

Call us on 9375 3902 if you'd like us to assess your audio tapes. 

Can you repair a CD Master with Distortion?

It's not always old analogue recordings that have problems. Digital recording has it's own issues and can highlight problems in the recording process even more than older analogue recordings that sometimes mask issues.

In this example we were provided a CD master for production that was digitally recorded, mixed and mastered and at some point problems have been introduced.

These problems with the CD master were first detected when we analysed the master prior to production, and after the analysis process we delved further in to the recording to see if we could do anything simple to rectify the problem.

The initial problem indicated E32 & Tracking Loss errors on the CD master and as these are considered "fatal errors" the production of bulk copies won't go ahead with these types of errors on a master. Here's a screenshot of the analyser report. (Learn more about CD errors here)

Once we saw the error report we decided to look in to the problems further to see if it was a disc burning issue or whether something else may be causing the issues. The CD master was loaded in to our audio editing suite so we could have a closer look and several problems were immediately detected. 

As can be seen on the following image this track had severe distortion on the left hand channel towards the end of the piece.  

The corresponding audio can be heard here, The audio problem sounds a bit like a faulty speaker on the left hand channel. 

This sound is usually caused in digital recordings when the safe operating levels are overloaded at some stage in the process causing the signal to be "clipped". It's a rather unpleasant sounding distortion for most people. 

The issue can be seen here on the zoomed in waveform, The flat top to the waveform is what is heard as distortion.

So notes were made and it was decided to process the audio to reduce this distortion rather than re- record, remix and re-master the CD. 

Unfortunately this wasn't the only problem, The horizontal line at the top of the right hand channel in the spectrograph below also caught our attention. Was it meant to be there? What exactly was it and what was causing this? 

Zooming around the area the cause of this anomaly is found. There is a series of oscilations above and below 17KHz. This type of problem can sometimes be caused with a faulty microphone cable where the earth connection or shielding may not be as good as possible and this allows electro-magnetic interference in to the recording chain.

While a lot of people can't hear these high frequency tones (they might impress your dog though!) it's better for them not to be in the audio so a series of filters was set up to remove them.

Checking other tracks on the CD and it was found there were other tracks with similar problems on both the left and right hand channels, some clicks and some more random distortion sounds, however the process used to remove them was pretty similar and the descision was made to repair the audio for each track and recompile. 

Here's an audio sample of the processed audio with the distortion reduced and oscilations removed.

And the waveform now shows more dynamics (variations in volume) where the edge of each chord has been restored by replacing the clipped portion of the waveform with a mathematical recreation of what should have been there in the first place.

After processing all the audio a new DDP master a CD master and Reference copy was provided to the client for approval prior to replication of the final CD copies.  You can read more about CD Mastering at ProCopy here:

This was quite a challenging repair job as the problems took a while to determine and process, however it was interesting to discover that digital recording isn't without potential problems if care isn't taken throughout the whole process. 

If you have any questions please don't hesitate to call us on 1300 4 PROCOPY or 08 9375 3902.