DISCLAIMER:
Any information found on this page is to be used for education only!
Please do not use this info for commercial use.
This page is started because of lack of awareness of the Video Cassette Recorder
(VCR) time-base errors.
I also used this information to build a VCR simulator.
During my professional carrier I was personally involved in the problems that the
time base errors caused in a TV system.
The problem got even bigger when the digital video processing found its entrance.
Before I will explain the origin of the problems I first explain some VCR basics.
There are some pages on the web that give you also information about this subject:
The line synchronization defines the start of a picture line.
1.1.2. Field Synchronization
The function of the field sync. is to start a field. The standard field
synchronization also holds line sync information. The function of the egalisation
pulses in front and back of the field sync. is to egalise the field synchronization
detection. The traditional way of detecting the field sync. is by using an integrator
(see Figure 4).
One complete PAL (Phase Alternate Line) picture consists of 625 lines.
The frequency of this picture is 25Hz. The picture frequency of a film is also
25Hz, this not a coincidence. But for TV a field frequency of 25Hz would flicker
too much. This is why the picture is spit up in 2 fields, odd (Figure 5) and even
(Figure 6) of 312.5 lines that are displayed in between each other (interlined).
The frequency of a field is 50Hz.
In Figure 7 you can see a complete frame (=2 fields).
If a conventional way of recording is used like the audio cassette recorder
(20Hz-20kHz) to record a video signal (1Hz-3MHz) then you should need a
tape of 3000mtr for 10minutes recording at a tape speed of 5m/s. This is
not efficient.
For this reason the helical scan was introduced. See Figure 8. The
first system had two reels on a different height.
This way 10 minutes of recording will use 13.2mtr of tape with a relative tape
speed of 5m/s. Much more efficient.
Tracks are written diagonal on the tape next to each other.
For the VHS system a cassette is used where the tape reel has no height difference.
The Head-drum is placed in an angle and the tape moves along the head-drum. See
Figure 9.
For PAL the number of revolutions per minute is 1500rpm.
On the head-drum 2 (or more) video heads are placed in exactly 180º opposite (more about
multiple heads later). In Figure 10, the bottom view of the head-drum has is drawn. There
are 4 video heads A, B’ and B, A’.
One video head writes one complete field (312.5 lines) with some overlap. This overlap
is necessary for having no signal loss when switching the video heads at the field switch
in all modes (playback, still and scan).
Figure 10: Top view of head-drum
In Figure 11 you can see the track layout. The head switch is (in this example) positioned,
8 lines before the field sync. According to the ICE 756 (935kB) standard
the head switch is allowed 0 to 10 lines in front of the vertical sync. Zero lines in front of
the vertical sync are only allowed, when absolutely no disturbance in the synchronization occurs.
The overlap (in this example) is 6 lines long.
Figure 11: The track layout
2.1. Azimuth
To use the tape efficient and to reduce cross talk between tracks on the tape, the
tracks have a different azimuth. One video head has an azimuth "A" the
other has azimuth "B" (see Figure 12).
Figure 12: Azimuth Video head A and B
Video head A is not able to read video track of head B. This way cross-talk between the
tracks is avoided so they can be positioned close together.
Figure 13: Video Head A and Video Head B
Figure 14: Video Head A is not able to read track B
2.2. Video head Drum
2.2.1. 2 head VCR
1 pair of video heads A and B are positioned exactly 180 degrees in opposite of each
other. See Figure 15.
Figure 15: 2 Head drum
2.2.2. 3 head VCR
On the 3 head drum, 1 pair of video heads A and B, are positioned exactly 180 degrees
in opposite of each other. One extra head B’ is added. This head has the same azimuth
as head B, so it is able to read the tracks of head B.
This extra video head will make it possible to read only one track in case of the
trick mode still picture. See Figure 16.
Video head B’ is placed next to video head A. The distance is in such a way that the line synchronization
will have an exact number of line distance. This makes track scan possible.
Figure 16: 3 Head drum
2.2.3. HiFi Audio VCR
When audio is recorded longitudinal as the tape recorder phase problems occur. It is
impossible to record Dolby Surround sound. For recording this, phase should be highly
very constant.
Therefore the audio is recorded with rotating audio heads (See Figure 17). These heads
are placed on the video drum with an angle of 120
°
to the video heads. The azimuth of the audio head C is
+30° and of head
C’ is -30°.
The left and right audio channels are modulated on 2 FM
modulated frequencies. Some audio processing at playback will hide the rattle
distortion caused by the switching of the audio heads.
Figure 17: HiFi Audio recording
First the Audio heads are writing their track deep in the tape (See Figure 18).
Then the video heads write the video tracks on the surface less deep.
Figure 18: Deep audio recording
2.2.4. 4 head VCR
On the 4 head drum (Figure 19) 2 pairs of video heads are positioned. Video heads
A and B are positioned exactly 180°
in opposite to each other. Next to head A and B there are
head B’ and A’ with a distance of whole number of lines. Head A and A’ have the same
azimuth and Head B and B’ have the same azimuth. The reason will soon become clear.
Figure 19: 4 video head
2.3. Cryterium and tape speed
The cryterium is the number of lines offset there is between 2 adjacent tracks.
In Table 1 you can see some cryterium numbers.
NTSC
PAL
VHS
8mm
VHS
8mm
SP
LP
EP
SP
LP
SP
LP
SP
LP
1.5
0.75
0.5
1
0.5
1.5
0.75
2
1
Table 1: Cryterium table
Now we have reached an interesting subject: sync alignment. There is alignment
when the line synchronization from track to track is "in line".
The Cryterium is one of the most important parameters together with the system (PAL, NTSC)
and recording speed.
In Figure 20 the Guard band, track width and cryterium is given for the Standard speed
(SP). There is no overlap (less cross talk) and you can see there is sync alignment.
Figure 20: SP Recording speed and cryterium
In Figure 21 the Long Play (LP) speed is illustrated. The speed is halve the speed of SP. Now there
is an overlap of 1m
m, this causes more cross talk than in SP. The cryterium is 0.75 lines and you can see that
there is NO sync alignment. Between 2 adjacent tracks there is a sync jump of
16m
s.
Figure 21: LP Recording speed and cryterium
Super long Play (SLP) or Extended Play (EP) is only used in NTSC see Figure 22.
There is an overlap of 10.7m
m and even more cross talk than in LP. The cryterium is
0.5 lines and there is sync alignment.
Figure 22: EP (SLP) Recording speed and cryterium
2.4. Playback
Until now we have listed some dull facts. But now it starts to get interesting.
When a VCR is in playback mode the separate tracks are scanned and displayed. During playback
the following errors will occur:
Head switch disturbance
One Head error
Two Head error
Tape transport jengle
These errors are also present during all other modes.
2.4.1. Head switch
When there is absolutely no switch disturbance, the position may even be 0 to 10 lines in front
of the field synchronization. In practice there is disturbance, so the switching is somewhere
between 5 to 10 lines in front of the field synchronization. (See Figure 23)
It is possible that one line synchronization pulse is distorted or it can even completely
disappear. It is also possible that due to the distortion an extra line is counted. This is
important to remember:
never trust the count
of picture lines in case of VCR signals
In Figure 24 the head switch causes a glitch that could cause a synchronization
distortion.
The head switch could also be positioned at the start of the vertical synchronization,
zero lines in front of the vertical synchronisation (See Figure 25).
Then the VCR replaces the last couple of line synchronization by artificial line
synchronization. They are synchronized to the original line sync. An artificial
vertical synchronization is inserted (later more). This sync pulse holds no line
synchronization.
The relation between vertical and horizontal synchronization is not fixed.
This makes it difficult to identify whether
the field is odd or even.
.
Figure 25: Head switch at vertical synchronization
Due to the tape stretch (or schrink) the line frequency of the scanned video could be lower (or higher).
The VCR will control the drum rotation speed in such a way that the frequency is held close to nominal
(0.2% for 625/50).
2.4.1.1. One Head error
Due to tape stretch, the video head scan could be shorter than the track information on
the tape. The tape stretch or shrink can be caused by:
Tape tension
Tape age
Video Head drum diameter (different on other VCR)
In Figure 26 the tape is stretched, each scanned field is "a" time short.
This results in a phase jump at the video head switch. This phase jump is in one
direction this is why it is called "one head error".
In the ICE 756 (935kB) standard the maximum allowed phase jump
for a 525/60 system is ±6µs. For the 625/50 system the maximum phase jump is ±15µs.
When VCR tapes are copied -without time base correction- the phase jump can increase
even above ±15µs.
Figure 26: One head error
Figure 27: TV picture One head error
In Figure 27 the top picture shows a normal TV picture. No disturbance is visible.
The bottom picture is a professional monitor with vertical and horizontal synchronization
delay. This picture clearly shows the phase jump.
2.4.1.2. Two Head error
The two-head error is caused by the head positioning error. In Figure 28 the video heads A
and B are not placed exactly 180° in opposite of each other, the scan of head A will have
be different than the scan of head B. The difference is "a". This error will
only be visible when the recording is played in a different recorder.
At a relative tape speed of 5m/s, and a 5µm placement error results in 1µs 2 head
error.
Figure 28: Head positioning error
Figure 29: 2 Head error
2.4.1.3. Artificial Vertical Synchronisation
During trick modes, but there are exceptions (Read exceptions), the vertical
synchronization pulse is replaced by a single pulse with a duration of 2.5 lines to 10
lines. The position of the leading edge shall be 0 to 10 lines in front of the vertical
sync. When there is absolutely no sync. disturbance, the position may be 0.
The period time between the artificial pulses may be changed to improve picture
stability (See Figure 30).
Figure 30: Duration and position of artificial vertical sync
2.5. Still Picture
The VCR "Still picture" mode is a so called trick mode. In this mode the tape speed is
zero. The drum rotation speed is controlled in such a way that the line frequency is
held close to nominal (2%). This is a must for the television receiver because in the television
colour decoder, the PAL line delay is a fixed delay line.
The sacn angle is different then in playback mode. The number of lines is increased by
the cryterium number, for each field. So for a complete frame the number of lines is increased
by 3.
You can calculate the number of frame lines with the formula:
Number of lines = 628 = 625 - (n-1) *2c
Where:
n = tape speed.
c = cryterium
2.5.1. SP 2 head
In figure 31 the scan of both video heads is given. In case of a 2 head VCR 2 tracks are scanned. This will
result in a "NOT perfect still picture". When the 2 fields have a different contents (moving object),
the difference will be visible.
The scan of video head A starts at the middle of line 307 and ends at the middle of line 621.
The scan of video head B starts at the middle of line 618 and ends at the middle of line 307.
For PAL SP there is sync allignment so there will be NO synchronisation phase jumps at the head switch.
Head switch A to B switches in the middle of line 621 to the middle of line 618. No phase jump.
Head switch B to A switches in the middle of line 307 to the middle of line 307. No phase jump.
Figure 31: Still Picture SP 2 head
2.5.1.1. SP 2 head (VR231)
The VCR VR231, is low-end VCR that is able to record in SP and LP. It uses 1 video
head pair. The video head width is optimized for SP and LP. This results in a big
noise bar.
In figure 32 you can see that one field is completely disturbed by noise.
You can also see that one-head error is also present.
Figure 32: Still Picture SP VR231
2.5.1.2. SP 2 head (VR241)
The VCR VR241, is a low-end VCR that only supports the SP record speed. The video heads
are optimized for SP and this results in a small noise bar. See figure 33.
Figure 33: Still Picture SP VR241
2.5.2. SP 4 head
In case of a 4 video head VCR only one track is scanned. Video heads A and A' or B and B' are
use (2.2.4. 4 head VCR).
The result is a perfect still picture.
Video head A starts at the middle of line 307 and end at line 621. See figure 34.
Video head A' starts at the middle of line 307 + k lines and end at line 621 + k lines.
The number k is a multiple of lines.
Head switch A to A' switches in the middle of line 621 to the middle of line 307+k.
No phase jump.
Head switch A' to A switches in the middle of line 621+k to the middle of line 307.
No phase jump.
Figure 34: Still Picture SP 4 head
2.5.2.1. SP 4 head (VR737)
The VCR VR737 is a high-end VCR with 4 video heads. The VCR is capable to record in SP
and LP.
Two video heads are optimized for SP and 2 for LP. In trick modes all 4 video heads
are used.
In figure 35 you can see a perfect still picture of only one field.
Figure 35: Still Picture SP VR737
2.5.3. LP 2 head
Until now we still have described SP. For PAL LP it is even more interesting. We have
learned that in PAL LP there is NO sync alignment (2.3. Cryterium and
tape speed), so this will introduce some new synchronization errors.
Video head A starts at the middle of line 307 and ends at ¾ of line 620.
Video head B starts at the ¼ of line 619 and ends at middle of line 307.
Head switch A to B switches at ¾ of line 620 to ¼ of line 619. Here we have
a
phase jump of +32µs.
Head switch B to A switches in the middle of line 307 to the
middle of line 307. No phase jump.
This means that every 2 fields there is a phase jump of 32µs. See figure 36.
Due to this phase jump a television receiver can have a problem with synchronizing to
the signal at the top of the screen.
Figure 36: Still Picture LP 2 head
2.5.3.1. LP 2 head (VR231)
The VCR VR231, has 2 video heads so it scans 2 tracks.
In figure 37 you can clearly see that there is a phase jump of 32µs every 2 fields.
You will also notice that the picture has no colour. This is because of the wrong PAL
phase at the head switch.
Figure 37: Still Picture LP VR231
2.5.4. LP 4 head
The 4 head VCR only scans 1 track.
Video head A starts at the middle of line 307 and ends at ¾ of line 620.
Video head A' starts at the middle of line 307+k and ends at ¾ of line 620+k.
Head switch A to A' switches at ¾ of line 620 to ¼ of line 307+k.
Here we have a +16µs phase jump.
Head switch A' to A switches at ¾ of line 620+k to ¼ o line 307.
Here we have again a +16µs phase jump.
So every field there is a phase jump of +16µs.
Figure 38: Still Picture LP 4 head
2.5.4.1. LP 4 head (VR737)
In figure 39 you can clearly see the +16µs phase jump at every head switch.
Only one track is scanned and there is no colour because of the wrong PAL phase at the head switch.
Figure 39: Still Picture LP VR737
2.6. Scan Forward
The VCR "Scan forward" mode is another trick mode. In this mode the tape speed is faster in
playback direction. The drum rotation speed is controlled in such a way that the line frequency
is held close to nominal (2% for 625/50).
The scan angle will be different than in playback.
You can calculate the number of frame lines with the formula:
When the tape speed is +3 times the normal playback speed the number of line in a complete frame is
decreased to 619 lines.
Number of lines = 619 = 625 - (n-1) *2c
Where:
n = tape speed.
c = cryterium
2.6.1. SP 2 head
In figure 40 the scan of a 2 head VCR is given. Head A scans from the start of track n
(1) to the end of track n+2 (2). Head A is not able to read track n+1. During the time
track n+1 is passed, noise will be visible. The length of the noise bars is depending
on:
Video head width
Recording speed (SP, LP)
Head B scans from the start of track n+3 (3) to the end of
track n+5 (4). Head B is not able to read track n+4. When track n+4 is scanned by head B again noise will be
visible.
You can see that every field scan 3 lines are skipped (2 times the cryterium).
Figure 40: Scan Forward SP 2 head
2.6.1.1. SP 2 head (VR231)
The 2 head VCR VR231 is optimized for playback of SP and LP. The video head width is NOT optimal for SP, so
in figure 41 you can see the big noise bars.
The VCR is in scan forward 11x.
Figure 41: Scan Forward SP VR231
2.6.1.2. SP 2 head (VR241)
The 2 head VCR VR241 is optimized for playback of SP only. The video head width is
optimal for SP. In figure 42 the noise bars are less width than in figure 41.
The VCR is in scan 11x forward.
Figure 42: Scan Forward SP VR231
2.6.2. SP 4 head
The 4 head VCR VR737 uses all 4 heads in the scan mode. When video head A scan a track
of the other azimuth is not able to read, the VCR switches to
video head B'. Then no noise will occur. But due to the switching, 2 extra head
switches are visible.
Figure 43 will be added when available.
2.6.2.1. SP 4 head (VR737
The VR737 4 head recorder scans all tracks but because it is switching to video
heads A' and B' it has twice as much head switches.
Figure 44: Scan Forward SP VR737
2.6.3. LP 2 head
For PAL LP recordings there is no sync alignment2.3.. When a video
head scans from track to the next track, a phase jump will occur in the line
synchronization. In figure 45 you can clearly see that the line synchronization is
not in line from track to track.
Video head A will scan from track n (1) to track n+2 (2). Video head A is not able to
read track n+1, so noise will be visible during the scan of the track. The noise bar is
short. This is because the tracks are close to each other in LP.
The phase jump from track n to n+2 is 32µs.
Figure 45: Scan Forward LP 2 head
2.6.3.1. LP 2 head (VR231)
In figure 46 the VCR is in scan forward +11 mode. At each track crossing a phase jump of
+32µs will occur. In this case there are 4 phase jumps of 32µs. Not many
synchronization circuits handle this quite well.
Figure 46: Scan Forward LP VR231
2.6.4. LP 4 head (VR737)
In figure 47 the 4 head VCR VR737 will switch from video head A to B' when it crosses
track n+1 (See figure 45). There will be twice as much of head switches and phase jumps.
But the phase jumps have a smaller amplitude. At every headswitch (track swap) the
phase jump is now +16µs.
jumps
Figure 47: Scan Forward LP VR737
2.7. Scan Backward
2.7.1. SP 2 head (VR231)
Figure 48: Scan Backward SP VR231
2.7.1. SP 2 head (VR241)
Figure 49: Scan Backward SP VR241
2.7.2. SP 4 head (VR737)
Figure 50: Scan Backward SP VR737
2.7.3. LP 2 head (VR231)
Figure 51: Scan Backward LP VR231
2.7.4. LP 4 head
Figure 52: Scan Backward LP VR737
2.8. Chrominance
The luminance information bandwidth is 3MHz and the chrominance bandwitdh is 1MHz.
The luminance information is FM modulated on a 3.5MHz carrier. The chrominance is converted to a lower
frequency. This convertion is done in such a way that the line frequency changes at playback are used in
the chrominance convertion to corrected chrominance errors.
In figure 53 you can see the vectorscoop image of a chrominance signal from a PAL VCR.
Figure 53: Vector scoop VCR Play
In figure 54 you can see the vectorscoop image of a chrominance signal from a PAL test pattern generator.
Figure 54: Vector scoop Generator
2.9. Macrovision
Macro vision introduced around 1986 a copy guard (anti-dubbing) methode, the so called JVC/Macrovision VHS-23
Standard.
The standard is based on disturbing the video AGC (Atomatic Gain Control), of the VCR when it is in recording mode.
The macrovision signal should not disturbe the VCR in playback mode and TV displaying the signal.
The VCR RF/IF is like the RF/IF of the TV. The IF AGC will make sure that the video top synchronization level
to the blanking level is held constant. The TV and VCR have also CVBS input(s). At this point the VCR has a video
AGC. The TV has not. The video AGC holds top synchronization to blanking levels constant. For this control the top
synchronization and the blanking level is measured. The blanking level is measured at the colour burst position.
I will not go into detail about the Macrovision signal. Later I will add more figures.
There are 3 kind of Macrovision signals:
Back Porch Pulse
Pseudo sync pulse
Colour stripe
2.9.1. Back Porch Pulse
The Back Porch pulse is used for PAL and NTSC, VCR and DVD. In figure 55 you can see 14 lines with each one
Back Porch pulse.
The lines are in front of the vertical synchronisation. The main target of this pulse is to disturbe the video AGC
in such a way that the VCR is not able to detect the vertical synchronization correct. The blanking sample level
is at top white level, so the video AGC will decrease the CVBS signal and the VCR is not able to synchronize to the
signal.
The amplitude of the pulse is constant.
I will add some new drawing soon.
Figure 55: Back Porch pulses (14)
2.9.2. Pseudo sync pulse
In figure 56 you can see 10 lines with each 6 Pseudo-sync pulses. The lines are positioned in the vertical blanking.
The main target of the pulses is to disturbe the video AGC in such a way that synchronisation at the top of the
screen is impossible. To make the distortion irregular, the amplitude of 2 pairs of pulses are controlled different.
The Pseudo sync pulse is used for PAL and NTSC, VCR and DVD.
Figure 56: Pseudo-sync pulses Macrovision (10)
2.9.3. Colourstripe
The Macrovision Colourstriping is only used on DVD and (only) NTSC. Colourstriping is based on inverting the
colour burst of a group of 4 succesive lines. Colour striping is not allowed for PAL.
It would be impossible to use it on VCR tapes because the precession that is needed to correct the burst
phase. We have learned that synchronization on VCR signals is a problem at it self.
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