lecture #6: color tv interlacing fields€¦ · • our eyes average this out on a black and white...

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Lecture #6: Color TV (Download Handout from Web Page)

Interlacing Fields:

Interlace Raster Scan

Illustration of scanning beam on CRT face during Interlace scan.

Interlaced Scan

Illustration of how Interlacing is accompolished.

(Even) (Odd)


Raster Scan Deflection Waveforms

Figure shows Horizontal & Vertical Sweep Voltage Waveforms, and their relative timing Relationships.

• Odd Fields start in the upper left corner and end in the bottom middle. • Even Fields start in the upper middle and end in the bottom right. • What is wrong with this picture?

o Only 10 lines/Field. o Vertical Retrace time should be 0.95ms (p.192 of Handout). That is the

time it takes to do 15 horizontal retraces.

247.5

495


Horizontal Sync Complex

Illustrating "Front Porch," "Sync Tip," "Back Porch" with Color Burst riding on it. The interval between the end of the sync tip & the start of the burst is sometimes refered to as the "Breeze -way." Note also, the "Burst Flag" (D.C. Restore clamp).

• 1 line = 63.5 us → Horizontal Refresh Rate 15750 Hz. • Horizontal Retrace Time = 1.5us + 4.7us + 4.7us = 10.9 us.


Raster Scan Synchronization Waveforms

NTSC/RS-170A, complete set of waveforms, including Composite Blanking. Note Color Subcarrier phasing (in red).

http://www.ntsc-tv.com/images/tv/RS-170A.gif


Waveforms from File: 8Bars.20MHz.bin

• Field 1 o Ends on a ½ line in the bottom middle of the screen

• Field 2 o Identify Odd or Even Field by finding the starting point for 6 high pulses

of ½ line duration followed by 6 low pulses of ½ line duration. o Vertical Retrace for 15 lines. o Starts on a ½ line o 247 ½ lines o Ends in the lower right corner o 15 + 247.5 = 262.5 lines * 63.5 us / line → 60 Hz Refresh for even fields.

Field 1 Start of Field 2 – Vertical Blanking Interval (15 lines)


• Field 2

o Ends on a full line in the lower right corner of the screen. • Field 1

o Vertical Retrace for 15 lines. o Starts on a full line o 247 ½ lines o Ends in the bottom middle of the screen o 15 + 247.5 = 262.5 lines * 63.5 us / line → 60 Hz Refresh for odd fields.

• 30 Hz refresh rate for interlaced image.

Field 2 Start of Field 1 – Vertical Blanking Interval


Quadrature Amplitude Modulation Trig Identities

( ) ( ) ( ) ( ) ( ) ( )( )

( ) ( )

( ) ( ) ( ) ( ) ( ) ( )( )

( ) ( )θφθφ

φθ

θφθφ

φθ

θφθφθφθφφφθθ

θφθφθφθφφφθθ

−++=

−+−=−+=

−++=

+++=++=

−−−+−+−−

−−−+−+−−

sin2

1sin

2

1

4

1

22sincos

cos2

1cos

2

1

4

1

22coscos

jjjjjjjj

jjjjjjjj

eeeejj

eeee

eeeeeeee

Modulation

( ) ( ) ( ) ( )φθφθ sincoscoscos 2211mod AAx +=

Demodulation – Cosine Channel

( ) ( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) ( )

( )11cosmod

2222111111

2211

222

11

2211modcosmod

cos

:Filter Pass LowAfter

2cos2

12sin

2

12cos

2

12cos

2

1cos

2sin2

1cos22cos

2

1

2

1cos2

sincoscos2coscos2

sincoscos2coscoscos2cos2*

θ

θφθφθφθφθ

φθφθ

φφθφθφφθφφθφ

Ax

AAAAA

AA

AA

AAxx

lpfde

de

=

−+++−+++=

+

+=

+=

+==

−−

−

Demodulation – Sin Channel

( ) ( ) ( ) ( ) ( ) ( )

( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) ( )

( )22sinmod

2222221111

2211

22211modsinmod

cos

:Filter Pass LowAfter

2cos2

12cos

2

1cos2sin

2

12sin

2

1

2cos2

1

2

1cos22sin

2

1cos2

sincos2cossincos2sin2*

θ

θφθφθθφθφ

φθφθ

φθφφθφ

Ax

AAAAA

AA

AAxx

lpfde

de

=

−−+−+−++=

−+

=

+==

−−

−


Spectrum of Composite Video:

• Spectrum originally designed for Black and White TV only. Only the Luminance

Signal was transmitted. • Designers needed to modify the TV signal in a way that didn’t affect existing

B&W TVs and added color to new Color TVs without increasing the bandwidth. • A lot of stuff in limited bandwidth

o Luminance bandwidth is 0 – 4.2MHz – This is the original Black and White Signal.

o I-Chrominance bandwidth is 0 – 1.6MHz o Q-Chrominance bandwidth is 0 – 0.6MHz

• Color (Chrominance) had to be added without affecting the operation of the black and white televisions.

• Chrominance bandwidth could be less than the Luminance bandwidth because our eyes can’t tell the difference. (Somebody smart figured that out).

• I-Chrominance Amplitude Modulates the cosine channel of the 3.579545 MHz Color Sub-Carrier.

• Q-Chrominance Amplitude Modulates the sine channel of the Color Sub-Carrier. • The frequency of the Color Sub-Carrier was picked to be the 227.5 times the

Horizontal Sweep Frequency. • The Chrominance looks like it would interfere with the Luminance. However, the

eye is not sensitive to this interference because of the “frequency-interlacing” effect. So a Black and White TV will still work with a Color TV signal.


o A color TV can remove the chrominance from the luminance with a comb filter (Moving Average of length 2).

• The Luminance looks like it would interfere with the Chrominance. But the eye isn’t sensitive to this either because of the “frequency-interlacing” effect.


Decomposition of Composite Video:

( ) ( ) ( ) ( ) ( ) ( )ttmttmtmtm ccIccQLv ωω cos*sin* ++≈

(Approximately because top bandwidth of I chrominance is filtered out causes the mIHh term to appear. Don’t worry about that right now.) mL(t):

1. Low Pass Filter mv(t) and use comb filter to remove chrominance. mQ(t):

1. Band Pass Filter mv(t) from 2 – 4.2MHz, and multiply by sine, amplitude = 2

( ) ( )( ) ( ) ( ) ( ) ( )[ ] ( )( ) ( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )ttmttmtmttmtm

tttmttmttm

tttmttmtm

ttmttz

ccIccQQccLL

ccccIccQccL

ccccIccQL

ccvcc

ωωωωωωω

ωωωωω

2sin*2cos*2sin*

sin*cos2*sin2*sin2*

sin2*cos*sin*

sin2*)sin(2*)(

2.42_2.42_

22.42_

2.42_

2.42_

+−++=

++=

++=

=

−−

−

−

−

2. Final low pass at 600KHz eliminates double frequency terms. 3. We are left with mQ(t) and mL_2-4.2(t). 4. mL_2-4.2(t) gets bandlimited to 600KHz but doesn’t interfere with mQ(t) because

of the “frequency-interlacing” effect.


mI(t): 1. Band Pass Filter mv(t) from 2 – 4.2MHz, and multiply by cosine, amplitude = 2

( ) ( )( ) ( ) ( ) ( ) ( )[ ] ( )( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( ) ( ) ( )ttmtmttmttmtm

ttmtttmttm

tttmttmtm

ttmttz

ccIIccQccLL

ccIccccQccL

ccccIccQL

ccvcc

ωωωωωωω

ωωωωω

2cos*2sin*2cos*

cos2*sin*)cos(2*cos2*

cos2*cos*sin*

cos2*)cos(2*)(

2.42_2.42_

22.42_

2.42_

2.42_

++++=

++=

++=

=

−−

−

−

−

2. Final low pass at 1.6MHz eliminates double frequency terms. 3. We are left with mI(t) and mL_2-4.2(t). 4. mL_2-4.2(t) gets bandlimited to 1.6MHz but doesn’t interfere with mI(t) because of

the “frequency-interlacing” effect.


“Frequency-Interlacing” Effect in the Time Domain • The color sub-carrier is 227.5 times the horizontal sweep frequency. • This causes the phase of the color sub-carrier to flip 180

� for adjacent lines

(spatial phase reversal). • For example, shown below is the Color Sub-Carrier for 3 adjacent lines in the odd

field:

Line 1

Line 3 Line 1

Line 5 Line 3


• Since the carrier amplitude is +/- 1 on adjacent lines, the chrominance is multiplied by +/- 1 on adjacent lines.

• Our eyes average this out on a Black and White TV. • Averaging adjacent lines removes the Chrominace from the Luminance in the

Color TV. • The Color Sub-Carrier is also varies by 180

� frame to frame. That is, the phase of

the Color Sub-Carrier for Line1/Frame1 is 180� out of phase with the carrier for

Line1/Frame2 (temporal phase reversal). • For example, Line1/Frame2 starts 525 lines*227.5 periods/line = 119437.5

periods of the Color Sub-Carrier after Line1/Frame1. Line1/Frame1 is 180� out of

phase with the previous Line1/Frame2. • The figure below graphically shows this temporal and spatial phase reversals:


“Frequency-Interlacing” Effect in the Time Domain

• Let’s look at the spectrum for vertical bars on a Color TV to see what the “Frequency-Interacing” looks like.

• We will consider the case where the Luminance, I-Chrominance and Q-Chrominance signals are pure tones.

• For the Horizontals lines to repeat, these pure tones need to be harmonics of the horizontal sweep frequency. Why?

o N/Fo=63.5uS=1/Fh. So, Fo=N*Fh . • We will ignore the Sync Tip and the Color Burst for right now.


Luminance Signal:

• Let’s put the Luminance Signal at the 10th harmonic of the horizontal sweep

frequency (157.48 KHz, DC Offset = 0.5, Amplitude=0.1) • Multiply this by a square wave whose duty cycle is (63.5us - 1.5us - 4.7us - 4.7

us)/63.5us = 82.83% to create the Horizontal Sync Complex for the Luminance. • Multiplication in the time domain is convolution in the frequency domain.

• Luminance Signal (red trace) is a pure tone at 157 KHz. • 82.83% Duty Cycle Square Wave is the white trace.

• Convolution of the white and red traces above. • The harmonics of the square wave are centered around the pure tone now instead

of around DC. • These harmonics are still at integer multiples of the horizontal sweep frequency

since the pure tone was a harmonic of it. • Now picture the convolution of green trace with the 30 Hz square wave which is 0

during the vertical retrace and you get the black trace below:


Chrominance Signal:

• I-Chrominance Signal be 945 KHz sine wave (60th harmonic - White Trace

below, 0.2V Amplitude). • Q-Chrominance Signal be 394 KHz sine wave (25th harmonic - Red Trace below,

0.1V Amplitude). • The Green trace below is the Color Sub-Carrier which is 227.5 times the

Horizontal Sweep Frequency (Fh). • QAM the Color Sub-Carrier with the Chrominance Signal. This gives you peaks

at Fcc +/- the I-Chrominance Frequency and at Fcc +/- the Q-Chrominance Frequency (Blue trace below). The lower side band frequencies of the blue trace are at 227.5-60=167.5 * Fh and 227.5-25=202.5 * Fh. Note that the Upper Side band of the I-Chrominance signal has been filtered out to limit the bandwidth to 4.2MHz.

• Multiply this by a square wave with frequency Fh and duty cycle (63.5us - 1.5us - 4.7us - 4.7 us)/63.5us = 82.83% to create the Horizontal Sync Complex for the Chrominance. (Yellow trace above and below).

• Multiplication in the time domain is convolution in the frequency domain. • The convolution positions the harmonics of Fh between the harmonics of the

Luminance signal.


• Zoom in of harmonics of the near DC shows that they are interlaced between the

Luminance

• Now picture the convolution of yellow trace above with the 30 Hz square wave which is 0 during the vertical retrace and you get the red trace below:

• Interlacing in the Frequency domain makes it invisible to the human eye, so the Black and White TV will still work.


Using the Luminance and Chrominance signals from above, we get this picture (Test.20MHz.bin). ��


• Video Signal


• Decoded Luminance and Chrominance signals


• Red, Green, and Blue Signals.

References:

• www.ntsc-tv.com • Section 4.9 of Dr. Morley’s textbook

http://www.ntsc-tv.com/

lecture #6: color tv interlacing fields€¦ · • our eyes average this out on a black and white...

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