During February 2020, Cheadle DATA Recovery Ltd performed a recovery for Dave Masterman, producer for BBC Radio 5 Live Elis James and John Robins show. They had lost three podcast recordings due to data corruption on an SD memory card. This happened because the audio recorder lost power mid recording. This, and CDR's involvement afterwards, is explained on the radio show (listen below, or via YouTube). An emergency 24 to 48-hour service was provided by CDR with a successful recovery. Full technical details of the SD card failure and the associated data recovery techniques can be found at the foot of this webpage - it proved a challenging case due to interleaving audio data.
This is Dave. Dave came from Manchester to London to record Elis and I doing an escape room. Dave carried all the kit. Dave has a dislocated shoulder. Dave spent two hours with us escaping from a room. This is Dave realising it hadn’t recorded. #PrayForDavepic.twitter.com/s1kNdbgCsf
To follow on from my former friend’s kind tweet about today’s audio disaster (@nomadicrevery), I thought I may as well now share with you the corrupt audio we have as a result. This will play in my nightmares for the next five years. pic.twitter.com/Kl9qsQWvLZ
The above tweet is about the corrupted audio which the radio producer had extracted by his own data recovery attempt. It is not a sample rate problem but instead relates as to how the specific audio recorder stores data to the SD card.
The portable audio recorder in use was a Zoom H4n audio recorder. This has the capability to record up to four audio streams simultaneously. It can be used with personal radio microphones so users can move freely whilst their individual voices are recorded. In the production edit, the separate audio files are played back simultaneously so that each person has the same level of volume.
File Allocation Table (FAT) corruption after power loss
The data loss was reported due to lack of battery power and the audio recorder suddenly losing power. As a consequence of this, the File Allocation Table (FAT) had become corrupt. On inspection of the SD card, it was clear that there were no folder or file name entries associated with the required data. Full sector-by-sector scans could not locate any reference to the required files in the FAT. The FAT defines the folder and filename structure, but also (critically) defines the location of files by LBA values and deals with any splitting of files if these are not written in sequential blocks in the SD card. Without the FAT it was necessary to perform a 'RAW' recovery and to locate files by file type (e.g. .WAV, .MP3 etc).
Interleaved audio streams
The garbled high-speed audio which can be heard on the radio show is because the recovery attempt made by the show's producer failed to take into account that the three audio streams were interleaved within the raw data. Whilst their 'RAW' recovery found the start of each file it could not take into account the method in which data was written to the SD card by the Zoom H4n audio recorder. Below are examples of working audio files from the Zoom audio recorder, which show this interleave between files.
Removal of the interleave
To obtain normal audio playback it was necessary to take each of the "RAW" .WAV files in the recovery, and then write a script to separate the three streams of audio. Essentially the first 64 sectors of data were extracted to create audio 'stream 1', the following 64 sectors of data to create 'stream 2', and the third 64 sectors to create 'stream 3'. This process was repeated over and over again until each of the three audio streams were fully assembled and would playback normally.
Partial recovery of the third podcast audio file
In the radio show, you can hear how the producer tried to "Wake up" the SD card by copying some additional data to the SD card after the data loss. The golden rule of data loss/recovery applies here: if you lose data turn the device off and do not attempt to use it. The audio recordings they wished to recover had not been marked as 'allocated' within the FAT. Consequently, when new data was copied to the SD card it wrote to areas of the SD card which contained the required data. Once data has been overwritten with new data there is no method to reverse this. As a consequence, it was only possible to recover the first 15 minutes of audio associated with the third podcast recording.
CDR were extremely helpful. They responded quickly and efficiently taking me through the process at every stage. It was made clear from the beginning what to expect, their no data no fee promise and... I was consulted throughout the process. I would definitely recommend CDR to anyone who needs data recovery services.read more
11:02 30 Jul 21
Cheadle Data saved me as I had dropped by external hard drive. John was exceptionally efficient in contacting me and clearly explained the process which I could understand. The workflow made it... simple for me to keep a track on the work. Only two documents lost and received my drive back promptly with an amazing return delivery service. I would have absolutely no hesitation in recommending this service. AMAZING!read more
Dr Tariq Ahmad PhD
23:48 29 Jul 21
Didn't use these guys, so can't give 5 stars, but found the chap on the phone to be super-friendly and helpful, even recommending an apparent competitor! 👏👏👏