Method and signal processing device for converting stereo signals for headphone listening

• US 20020039421A1
• Filed: 09/28/2001
• Published: 04/04/2002
• Est. Priority Date: 09/29/2000
• Status: Active Grant

• Current Assignee: Nokia Technologies Oy (Nokia Corporation)
• Original Assignee: Nokia Mobile Phones UK Limited (Nokia Corporation)
• Inventors: Kirkeby, Ole
• Granted Patent: US 6,771,778 B2
• Time in Patent Office: Days
• US Class Current: 381/17
• CPC Class Codes: H04S 1/005 For headphones

Abstract

The invention relates to a method for converting signals in two-channel stereo format to become suitable to be played back using headphones. The invention also relates to a signal processing device for carrying out said method. According to the invention left direct path (L_d) and left cross-talk path (L_X) signals are formed from the left input signal (L_in), and correspondingly right direct path (R_d) and right cross-talk path (R_X) signals are formed from the right input signal (R_in), and further the left output signal (L_out) is formed by combining said left direct-path (L_d) and said right cross-talk path (R_x) signals, and correspondingly, the right output signal (R_out) is formed by combining said right direct-path (R_d) and said left cross-talk path (L_x) signals. The direct path signals (L_d, R_d) each are formed using filtering (1, 3) associated with first frequency dependent gain (G_d) and the cross-talk path signals (L_x, R_x) each are formed using filtering (2, 4) associated with second frequency dependent gain (G_x) and by adding interaural time difference (ITD) (5, 6).

Claims

1. A method for converting two-channel stereo format left (L) and right (R) channel input signals (L_in, R_in) into left and right channel output signals (L_out, R_out), in which method

left direct path (L_d) and left cross-talk path (L_X) signals are formed from the left input signal (L_in), and correspondingly

right direct path (R_d) and right cross-talk path (R_X) signals are formed from the right input signal (R_n), and

the left output signal (L_out) is formed by combining said left direct-path (L_d) and said right cross-talk path (R_x) signals, and correspondingly,

the right output signal (R_out) is formed by combining said right direct-path (R_d) and said left cross-talk path (L_x) signals,

which said left and right channel output signals (L_out, R_out) thereby become suitable for headphone listening, characterized in that

the direct path signals (L_d, R_d) each are formed using filtering (1, 3) associated with first frequency dependent gain (G_d),

the cross-talk path signals (L_x, R_x) each are formed using filtering (2, 4) associated with second frequency dependent gain (G_x) and by adding interaural time difference (ITD) (5, 6),

said first and second frequency dependent gains (G_d, G_x) are given a common substantially constant reference value below a first frequency limit (f_low),

said first frequency dependent gain (G_d) is given a substantially constant value significantly greater than said reference value, and said second frequency dependent gain (G_x) is given a substantially constant value significantly less than said reference value above a second frequency limit (f_high), where

said second frequency limit (f_high) is greater than said first frequency limit (f_low), and

said interaural time difference (ITD) is given a frequency independent constant value or alternatively a frequency dependent value.

2. The method according to claim 1, characterized in that

said first and second frequency dependent gains (G_d, G_x) are given both a value of one below said first frequency limit (f_low), and

said first frequency dependent gain (G_d) is given a value of 2, and said second frequency dependent gain (G_x) is given a value of zero above said second frequency limit (f_high).

3. The method according to claim 1, characterized in that said direct path signals (L_d, R_d) both are scaled by a first scaling factor (S_d) and said cross-talk path signals (L_x, R_x) both are scaled by a second scaling factor (S_x) in order to make the sum amplitude of the output signals (L_out, R_out) to substantially match the sum amplitude of the input signals (L_in, R_in).

4.he. The method according to claim 3, characterized in that the said first and second scaling factors (S_x, S_d) both are given a value of 0.5.

5. The method according to claim 1, characterized in that said first frequency limit (f_low) is given a value around 1 kHz and said second frequency limit (f_high) is given a value around 2 kHz.

6. The method according to claim 1, characterized in that the interaural time difference (ITD) is given value/values below 1 ms.

7. A signal processing device (BSWN) for converting two-channel stereo format left (L) and right (R) channel input signals (L_in, R_in) into left and right channel output signals (L_out, R_out) suitable for headphone listening, characterized in that the signal processing device (BSWN) comprises at least

first filtering means (1) associated with first frequency dependent gain (G_d) to form left direct path signal (L_d) from said left input signal (L_in),

second filtering means (2) associated with second frequency dependent gain (G_x) in serial with first delay adding means (5) associated with interaural time difference (ITD) to form left cross-talk path signal (L_x) from said left input signal (L_in), associated with interaural time difference (ITD) to form left cross-talk path signal (L_x) from said left input signal (L_in),

third filtering means (3) associated with first frequency dependent gain (G_d) to form right direct path signal (R_x) from said right input signal (R_in),

fourth filtering means (4) associated with second frequency dependent gain (G_x) in serial with second delay adding means (6) associated with interaural time difference (ITD) to form right cross-talk path signal (R_x) from said right input signal (R_in),

first combining means (7) to form the left output signal (L_out) by combining said left direct-path (L_d) and said right cross-talk path (R_x) signals, and correspondingly,

second combining means (8) to form the right output signal (R_out) by combining said right direct-path (R_d) and said left cross-talk path (L_x) signals, and

said first and second frequency dependent gains (G_d, G_x) having a common constant reference value below a first frequency limit (f_low)

said first frequency dependent gain (G_d) having a substantially constant value significantly greater than said reference value, and said second frequency dependent gain (G_x) having a substantially constant value significantly less than said reference value above a second frequency limit (f_high), where p1 said second frequency limit (f_high) is greater than said first frequency limit (f_low), and

said interaural time difference (ITD) is having a frequency independent constant value or alternatively a frequency dependent value.

8. The signal processing device (BSWN) according to claim 7, characterized in that

said first and second frequency dependent gains (G_d, G_x) have a value of one below said first frequency limit (f_low), and

said first frequency dependent gain (G_d) has a value of 2, and said second frequency dependent gain (G_x) has a value of zero above said second frequency limit (f_high).

9. The signal processing device (BSWN) according to claim 7, characterized in that the direct paths (L_d, R_d) each comprise first scaling means (9, 11) associated with a first scaling factor (S_d) and the cross-talk paths (L_x, R_x) each comprise second scaling means (10, 12) associated with a second scaling factor (S_x) in order to scale each path to make the sum amplitude of the output signals (L_out, R_out) to substantially match the sum amplitude of the input signals (L_in, R_in).

10. The signal processing device (BSWN) according to claim 8, characterized in that said first and second scaling factors (S_d, S_x) both have a value of 0.5.

11. The signal processing device (BSWN) according to claim 7, characterized in that said first frequency limit (f_low) has a value around 1 kHz and said second frequency limit (f_high) has a value around 2 kHz.

12. The signal processing device (BSWN) according to claim 7, characterized in that the interaural time difference (ITD) has value/values below 1 ms.

13. The signal processing device (BSWN) according to claim 7, characterized in that the signal processing device (BSWN) is a digital signal processor and/or digital signal processing network.

14. The signal processing device (BSWN) according to claim 13, characterized in that the first (1) and second (2) filtering means, and correspondingly the third (3) and fourth (4) filtering means are formed using a specific digital filter structure (41), in which filter structure the output of a linear phase low-pass filter (42;

52) is combined with the output of a parallel digital delay line (43;

53) having delay equal to the group delay of said low-pass filter (42;

53).

15. The signal processing device (BSWN) according to claim 14, characterized in that the first (1), second (2), third (3) and fourth (4) filtering means are implemented using reduced network structure (FIG. 6) based on performing two convolutions.

16. The signal processing device (BSWN) according to claim 13, characterized in that the input signals (L_in, R_in) are preprocessed using a method that performs decorrelation.