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.
I have a device that was encrypted and I ask John for his assistance, unfortunatelly it was impossible to retrieve the data but he was really clear and informative.
10:31 09 Mar 21
i have drive can detect but not spin & incorrect function message after choose initialize disk use MBR in disk management
14:14 03 Mar 21
I bought a 1TB memory stick on Amazon, following 10GB of data saving it stopped working. I discussed with my colleague in hospital, Parisa, who took the extra effort of finding someone to help. She... consulted a few experts and all of them suggested that I contact John Reid at Cheadle DATA Recovery. John diagnosed the problem. It was a counterfeit device and although it says 1TB it actually was a 16GB memory stick and was a fictitious product. John provided fair, thorough and professional advice and did not take any payment for this consultative service. I have purchased a 1TB disk from John and have been using it with absolutely no problems. I thank John immensely and would highly recommend him. I would also like to thank my colleague, Parisa, for her support in finding John.read more
15:33 02 Mar 21
We suffered a power cut that most probably led to a hard drive failure. This combined with my (admittedly) relaxed attitude to backing things up meant we lost years of data - mostly photos. I was... going to write this off and put it down to experience but was persuaded by my wife to look for someone who could recover the data. To cut a long story short John at Cheadle DATA Recovery was able to recover pretty much all we'd thought was lost. For no fee he analysed the failed drive to give us an idea of what could be recovered and told us the likely cost should we wish to go ahead which we did. We now have years of photos that we'd almost forgotten about to sort out! I would recommend his services to anyone who finds themselves in a similar situation.read more