<PAGE>

                                                                 Exhibit 10.9.1

                          FIRST AMENDMENT TO SUBLEASE

          This First Amendment to Sublease ("Amendment") dated as of November 1,
                                             ---------
1999, is entered into by and between Smith Industries Aerospace & Defense
Systems Inc., a Delaware corporation ("Landlord") and e.Digital Corporation
                                       --------
(f/k/a Norris Communications Inc.), a Delaware corporation and American
Technology Corporation, a Delaware corporation ("Tenant").
                                                 ------

                                   RECITALS

          WHEREAS, Global Associates, Ltd., a Virginia corporation ("Former
Landlord") and Tenant have entered into that certain Sublease Agreement (the
"Sublease") dated as of the 11th day of July, 1997 for a portion of the building
known as 13112 Evening Creek Drive South, San Diego, California 92128 (the
"Premises"); and

          WHEREAS, Landlord purchased certain assets from Former Landlord
pursuant to a bankruptcy proceeding captioned In re Global Associates, Ltd.,
                                              ------------------------------
United States Bankruptcy Court for the District of Maryland (Greenbelt
Division), Case No. 98-10216-DK, including, without limitation, all of the
Former Landlord's right, title and interest as tenant in and to that certain
Lease Agreement dated August 14, 1996 by and between Scientific Atlanta, Inc.,
predecessor-in-interest to Bedford Property Investors, Inc., a Maryland
corporation ("Prime Landlord"), as landlord and Former Landlord, as tenant, as
amended by amendment dated October 23, 1995, and amendment dated January 24,
1997, as confirmed by the Prime Landlord and Landlord pursuant to that Second
Amendment to Lease Agreement and Reinstatement of Lease dated as of August 15,
1998 (as amended and reinstated, the "Prime Lease"); and

          WHEREAS, Landlord and Tenant desire to amend the Sublease as
particularly set forth herein.

          NOW, THEREFORE, in reliance upon the foregoing and for good and
valuable consideration, the receipt of which is hereby acknowledged, Landlord
and Tenant agree and hereby amend the Sublease as follows:

                                   AGREEMENT

          1.   Definitions. All capitalized terms used and not otherwise defined
               -----------
herein shall have the same meanings ascribed to them in the Sublease.

          2.   Term. Paragraph 3(B) is hereby amended by deleting "July 31,
               -----
2000" where it appears and inserting in lieu thereof "July 31, 2002."

          3.   Premises. The definition of Premises is amended to add
               ---------
approximately 5,131 rentable square feet to the Premises (the "Additional
Space"), such that, as of the date of this Amendment, the Premises shall
comprise approximately 18,056 rentable square feet in the building, as such
Premises are outlined and shown on Exhibit B attached to this Amendment and by
                                   ---------
this reference made a part hereof and hereby replacing Exhibit B to the
                                                       ---------
Sublease. Tenant represents that it has inspected the Additional Space and has
found the same in satisfactory condition, subject to the completion of the
tenant improvements specified in Exhibit C attached hereto and made a part
hereof.
<PAGE>

          4.   Base Rent. Paragraph 4(A) is hereby deleted and replaced with the
               ---------
following:

     "(A) Tenant covenants and agrees to pay to Landlord an annual base rent
(the "Base Rent") in equal monthly installments, each in advance on the first
      ---------
day of each calendar month during the term of this Sublease in accordance with
the following schedule:

<TABLE>
<CAPTION>

                             Annual Rate per
                             Square Foot of                        Monthly base
    Time Period          Premises Rentable Area  Annual Base Rent     Rent
----------------------   ----------------------  ----------------  ------------
<S>                      <C>                     <C>               <C>
From August 1, 1997             $12.84              $165,957.00     $13,829.75
 through July 31, 1998

From August 1, 1998             $13.44              $173,712.00     $14,476.00
 through July 31, 1999

From August 1, 1999             $14.04              $181,467.00     $15,122.25
 through October 31,
 1999

From November 1,                $14.04              $253,506.24     $21,125.50
 1999 through July 31,
 2000

From August 1, 2000             $14.64              $264,339.84     $22,028.32
 through July 31, 2001

From August 1, 2001             $15.24              $275,173.44     $22,931.12
 through July 31, 2002
</TABLE>

          5.   Additional Alterations. Landlord shall perform the tenant
               ----------------------
improvement work to the Additional Space requested by Tenant and shown on
Exhibit B attached hereto and as specified on the plans and specifications
identified as Attachment No. 1. Tenant shall reimburse Landlord for the costs
              ----------------
incurred by Landlord to perform the tenant improvements, less costs paid by
Landlord for the backflow prevention device and fire door. Landlord and Tenant
hereby agree that the amount to be paid by Tenant to Landlord to reimburse it
for such costs is $41,480.00, together with such additional costs as Landlord
may incur because of (a) change orders or other modifications requested by
Tenant and approved by Landlord, and (b) costs incurred by Landlord to perform
certain pre-occupancy tenant work and improvements, including reconfiguration
and installation of modular furniture. Such amount shall be payable as follows:
$8,296.00 has heretofore been paid or shall be paid by Tenant to Landlord upon
Tenant's execution and delivery of this Sublease. The remaining $33,184.00 shall
be paid no later than December 9, 1999. Any additional costs incurred by
Landlord under (a) and (b) above shall be billed to Tenant and immediately due
and payable.

          6.   This Amendment and Landlord's and Tenant's obligations hereunder
are conditioned upon obtaining the written consent of the Prime Landlord to this
Amendment.

          7.   The obligations of Tenant under the Sublease shall be the joint
and several responsibility of all entities named herein.

                                       2
<PAGE>

          8.   Except as expressly modified herein, the Sublease shall remain
unmodified and in full force and effect. In the event a conflict exists between
the terms and conditions of this Amendment and the terms and conditions of the
Sublease, the terms and conditions of this Amendment shall control.

                                       3
<PAGE>

          IN WITNESS WHEREOF, Landlord and Tenant have entered into this
Amendment as of this date set forth above.



LANDLORD:                               TENANT:

Date: __________________________, 1999  Date: _________________________, 1999

SMITHS INDUSTRIES AEROSPACE &           e.DIGITAL CORPORATION (f/k/a/ Norris
DEFENSE SYSTEMS, INC., a Delaware       Communications, Inc.), a Delaware
Corporation                             Corporation

By:                                     By: /s/ ALFRED H. FALK
                                            ------------------
Name:                                   Name: Alfred H. Falk
Its:                                    Its: President


                                        AMERICAN TECHNOLOGY CORPORATION,
                                        a Delaware Corporation

                                        By: /s/ CON BROSNAN
                                            ---------------
                                        Name: Con Brosnan
                                        Its: President


                                       4