How many different versions can be obtained from one of cockpit voice recorder tape? The answer is: many. In the case of Smolensk crash as it stands we have 8 different versions of recordings from the cockpit voice recorder (CVR). Those versions differ in terms of content and length of the recording even up to few minutes.
Above: Cockpit voice recorder from the tragic April 10, 2010 flight in the protective case
History of black boxes from Tu-154M which crashed in Smolensk 10th April 2010 is strange indeed. Both black boxes have been found on the crash site almost immediately after the crash. In fact, journalist Wisniewski who came at the crash site few minutes after the crash recorded location one of the black box before even any security forces appeared. Black boxes were then taken to Moscow for readout and from here copies of the recordings were supplied to Polish authorities. Very soon problems and strange coincidences began to emerge. First, Polish side realised that each copy has different content and length of the recording. Chair of Polish investigation commission Minister Jerzy Miller had to travel to Moscow at least three times in order to obtain next copies of the recording. One copy missed 17.5 seconds of the record and contained signal from a mobile phone from a room where this copy was made. Another copies had different length of the recording. Below summary of differences for several copies made for different institutions:
Original black boxes were never returned to Poland. Copies of CVR were also investigated by the Polish Institute of Forensic Research in Krakow. According to their report the earliest copies made in April 2010 cannot be verified in terms of authenticity. The reason for it is that there is no information about of the mains (power line) hum. In consequence a date when this copy was recorded cannot be verified. Experts from Institute of Forensic Research stated that in such circumstances they did not even attempt to verify authenticity of this copy. Different copies were also investigated by another institution: Central Forensic Laboratory of the Police (CFLP) in Warsaw. Transcript from those copies was supplied to the former Miller commission – official investigation body.
But that was not the end of the story. In 2013, in spite of having already several different copies, Polish Chief Military Prosecution Office resumed efforts to obtain next copies from the cockpit voice recording. This task was undertaken by sound editor Andrzej Artymowicz. In firsts months of 2014 he recorded from the source tape kept in Moscow several other copies. Based on those copies he and a group of other people prepared a new transcript which totally differs from previous transcripts prepared by the Institute of Forensic Research and the Central Forensic Laboratory of the Police. In this new transcript there are phrases where pilots and passengers cheerfully discuss breaking the aviation law during drinking beer in the cockpit. Also, several key phrases were described completely different between older and new transcript. Artymowicz of course was aware about those stark differences between different transcripts. In the official report submitted to the Chief Military Prosecution Office Artymowicz claimed that all previous recordings made for the Institute of Forensic Research and the Central Forensic Laboratory of the Police must be classified as invalid. This surprising claim is the first case in history of airplane investigations where several copies of CVR recorded by few different professional bodies for official investigation committees were classified as invalid.
But what is a ground for such a so strong claim made by Artymowicz? He reckons that experts from the Institute of Forensic Research and the Central Forensic Laboratory of the Police made gross errors in the recording procedure. Namely, sample rate of 11,000 samples per second chosen by specialists from both institutions was insufficient in order to obtain good quality signal. Artymowicz used instead sample rate 96,000 samples per second and claimed that this simply trick has a massive beneficial effect on the quality of sound what in turn enabled almost completely rewrite transcript from the cockpit voice recorder.
When Artymowicz report was revealed in April 2015 journalists from the large Polish daily newspaper Gazeta Wyborcza tried to validate Artymowicz claim. They contacted Professor Peter French – UK’s experienced expert in the field of forensic speech and acoustics and a founding member of the International Association for Forensic Phonetics and Acoustics (IAFPA).  Answer from Professor French surprised journalists: as according to the manufacturer specifications original black box records voice from the cockpit in the frequency range 300-3400 Hz sample rate 11,000 samples per second is fully sufficient to obtain a good quality signal. Therefore using higher sampling rate will not improve quality of speech signal. That means copies recorded by Artymowicz have no advantage compared with previous copies recorded for professional institutions.
Professor French elaborated the subject by discussing Shannon-Nyquist sampling theorem where in order to capture all the information from a continuous analogue signal (which was recorded on CVR magnetic tape) sample rate must be at least twice greater than the highest recorded frequency. According to the manufacturer specification the black box device MARS-BM mounted in the Tu-154M plane records voice from the cockpit in the frequency range of the telephone bandwidth, i.e. from 300 to 3400 Hz:
Consequently, as the highest recorded frequency is 3400 Hz minimal sample rate required to fully reconstruct the signal is 6800 samples per second. That means sample rate 11,000 samples per second used by the Institute of Forensic Research and the Central Forensic Laboratory of the Police is not only sufficient to fully reconstruct the signal but has also a large security margin in case if recorded frequencies were in fact higher. Professor French quickly demystified Artymowicz bluff with the sample rate. And Artymowicz report itself adds more serious problems with “new” copies of CVR such as:
- time signal which must be an integral part of the recording was missing on the source
- quality of sound recovered from tapes was much worse compared with previous
- range of recorded frequency was much greater compared with manufacturer specification
- and even the original tape itself was not in agreement with manufacturer specification, namely passive and active sides of the tape were inverted
We will look into those problems in turn.
Missing time code
Cockpit voice recorder mounted in Tu-154M plane – device MARS-BM – is a four track analogue magnetic tape recorder. First two tracks (channels) on the tape record sound that comes to pilot and co-pilot headphones. Third track captures joint sound from 3 microphones from the cockpit. Fourth channel records in the form of binary coded decimal current time from the plane main clock. Copies recorded for the Institute of Forensic Research and the Central Forensic Laboratory of the Police did contain time code . Later copies recorded by Artymowicz did not. On the fourth channel only reference signal was captured without actual time code. Artymowicz assumed that provided by Russian hosts player cannot properly read a magnetic tape and that is the reason for the missing time code. So he requested a second player but to no avail: second device also could not read the time code from fourth channel of the tape.
Time code is integral part of the record embedded into magnetic tape in similar way as other channels. That means integrity of the record may have been compromised and the tape Artymowicz worked on may not have been the same tape previous institutions worked with. Lack of the record integrity was ignored and Artymowicz assumed that the tape is authentic and integral despite missing time code.
Poor quality of playback devices and recovered sound
Large part of Artymowicz report is devoted to explain how many problems he had with obtaining copies from old and weary Russians players. He used 2 players provided by Russians. First one generated significant harmonic noise. Second one suffered from poor tape guiding mechanism what caused degradation of the signal. Moreover, this player could not read channel one from the tape. Both recorders had different speed of tape transport mechanism what caused that it was impossible to obtain two copies with the same length of the record. As Artymowicz could not obtain integral copy in acceptable quality he decided to make several copies using both recorders and then merge different poor quality records from different copies and make one “master” copy. In spite of fact that Artymowicz could not obtain a good quality record and copies made by him were in worse state compared with previous ones he claimed that quality of the “merged” record he prepared far exceeds any previous attempt.
Unusual range of recorded frequency
According to the manufacturer specifications MARS-BM device records sound in the frequency range 300-3400 Hz which is a standard telephone band. Frequencies below 300 and above 3400 Hz are cut off by high-pass and low-pass filters. Artymowicz claimed that in fact the original tape from MARS-BM contains sound in much higher frequencies – up to 14 kHz. This claim is unsupported by any evidence except for the doubtful tape itself. Artymowicz had the ideal opportunity to easily prove this claim though. Namely, Polish authorities are still keeping twin version of Tu-154M airplane with exactly the same avionics equipment and recorders. If a test recording made by MARS-BM on the twin Tupolev did contain higher frequencies (above the manufacturer specifications) that indeed would prove Artymowicz claim. Yet, he did some microphone range measurements on the twin Tupolev but for unknown reason did not attempt to make a test copy on CVR recorder in order to compare technical characteristics of the signal with the characteristics from the tape kept in Moscow. Hence, as it stands we don’t know if the MARS-BM black box actually does not filter higher frequencies or Artymowicz was presented in Moscow with a tape not from the tragic flight (or the tape was manipulated).
Inverted sides of the tape
Normally, according to manufacturer specifications, half-inch magnetic tape used in MARSBM recorder has one side active (matte), where recording takes places, and the other side passive (glossy. During recording procedure in Moscow Artymowicz noticed that those sides are inverted: glossy side is active whereas matte side is passive. That raises further doubts whether this tape is the original tape from the tragic flight.
Number of different versions of contradictory copies and transcripts from the cockpit voice recorder strongly suggests that the original tape was manipulated. Hopefully, new Committee for Investigation of National Aviation Accidents, the body which has resumed investigation about Smolensk crash, will analyse also issues regarding recordings form the cockpit voice recorder.
Piotr Kublicki is an engineer with more than a dash of writer. He graduated in Poland with a degree in humanities and worked few years as journalist, editor and freelancer in the subjects of politics, social, religious and science. In 2000s he moved to the United Kingdom where he graduated with a degree in computing at the Plymouth University. He works in the areas of systems analysis and integration as well as software engineering.
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