Checking tampered audiotapes is a time consuming process: experts
Experts say the audiotape is run through a voice spectrograph machine where the graph peaks whenever the smooth flow of the tape is hampered showing signs of tampering, reports Satyen Mohapatra.india Updated: Mar 23, 2009 23:06 IST
There are at least four forensic laboratories in the country which specialise in investigating 'doctored' audio and videotapes. Controversial tapes like Varun Gandhi's maybe sent to any of these, depending on the investing agencies.
Principal Scientific Officer at the Central Forensic Science Laboratory, Rajinder Singh, speaks to Hindustan Times about the procedure involved in investigating these tapes. He said, "The facility for checking audio video tampering exists in forensic laboratories at Ahmedabad, Chandigarh, Hyderabad and at Central Forensic Laboratory Delhi."
"Your voice leaves a voice print which can identify you positively to a great extent," says Dr Rajinder Singh.
He adds, "We get lot of cases of audio tampering and do voice analyses at our laboratory. We listen to the tape critically and check it for background noise, discontinuity of speech, pauses or any abrupt stop. We also look for any disruption in the smooth running of the audiotape, if there has been any erasing or change in the background noise and atmosphere, or discontinuity even in a single word in the running speech.
The audiotape is also run through a voice spectrograph machine where the graph peaks whenever the smooth flow of the tape is hampered showing signs of tampering, he said.
The voice on a tape can be identified with the original person's voice depending on the taped voice's clarity, accent and distinctive features as well as recording conditions. The voice on the tape maybe disguised which is difficult to identify at times," he added.
Speaking on the similar issue, Senior Scientific Officer Gautam Roy at the Central Forensic Science Laboratory, which also specialises in investigating 'doctored' video tapes, said, "We need to see the original cassette or memory card as it is not possible at the stage of the CD to find out whether there has been any tampering because it is a second generation copy."
At the Chandigarh laboratory we have Video Measurement System or Non Linear Editing System in which we can see the tape frame by frame. Normally, 24 frames make for a one second film.
Any editing by cutting or joining of frames on the original tape can be seen on the screen, in this machine, as the graph on screen peaks if there is overlapping on the original tape, he added.
"When we run the picture frame by frame on this system we can see one frame becoming hazy and a new picture coming up if the tape has been doctored."
" It's an extremely time consuming and painstaking task, we run the tapes many times, pause and freeze frames viewing them as well as listening to audio to detect if there is any tampering by overlapping of frames."
For detecting audio tampering the audio part is generally extracted from the video if necessary and run through a voice spectrograph. At times of any addition and alteration the graph of the voice peaks, he added.