<PAGE>   1
                                                                   EXHIBIT 3.15


                   CERTIFICATE OF DETERMINATION OF PREFERENCES
                                       OF
                            SERIES N PREFERRED STOCK
                                       OF
                                   DISC, INC.


           We, J. Richard Ellis and Henry Madrid, hereby certify that we are the
President and the Chief Financial Officer, respectively, of DISC, INC., a
corporation organized and existing under the General Corporation Law of the
State of California, and further, DO HEREBY CERTIFY:

           That pursuant to the authority conferred upon the Board of Directors
by the Articles of Incorporation of the said Corporation, the said Board of
Directors on December 31, 1996 adopted the following resolution creating a
series of 666,667 shares of Preferred Stock designated as Series N Preferred
Stock, none of which shares have been issued:

                     "RESOLVED, that pursuant to the authority vested in the
           Board of Directors of the corporation by the Articles of
           Incorporation, the Board of Directors does hereby provide for the
           issuance of a series of Preferred Stock, no par value, of the
           Corporation, to be designated "Series N Preferred Stock," initially
           consisting of 666,667 shares and to the extent that the designations,
           powers, preferences and relative and other special rights and the
           qualifications, limitations and restrictions of the Series N
           Preferred Stock are not stated and expressed in the Articles of
           Incorporation, does hereby fix and herein state and express such
           designations, powers, preferences and relative and other special
           rights and the qualifications, limitations and restrictions thereof,
           as follows (all terms used herein which are defined in the Articles
           of Incorporation shall be deemed to have the meanings provided
           therein):

                     Section 1.  Designation and Amount. The shares of such
           series shall be designated as "Series N Preferred Stock," no par
           value, and the number of shares constituting such series shall be
           666,667.

                     Section 2.  Dividends and Distributions.

                               (A) Subject to the prior and superior right of
           the holders of any shares of Series C Preferred Stock ranking prior
           and superior to the shares of Series N Preferred Stock with respect
           to dividends, and pari passu with the rights of the holders of shares
           of Series D Preferred Stock, Series E Preferred Stock, Series F
           Preferred Stock, Series G Preferred Stock, Series H Preferred Stock,
           Series I Preferred Stock, Series J Preferred Stock, Series K
           Preferred Stock, Series L Preferred Stock and Series M Preferred
           Stock with respect to dividends, subject to the rights of any series
           of Preferred Stock which may hereafter come into existence, the
           holders of shares of Series N Preferred Stock shall be entitled to
           receive when, as and if declared by the Board of Directors out of
           funds legally available for the purpose, dividends in the same amount
           per share as declared on the




                                       55

<PAGE>   2

           Common Stock, treating such number of shares of Series N Preferred
           Stock for this purpose as equal to the number of shares of Common
           Stock into which it is then convertible. In the event any dividends
           are declared or paid on the outstanding shares of Series D Preferred
           Stock, Series E Preferred Stock, Series F Preferred Stock, Series G
           Preferred Stock, Series H Preferred Stock, Series I Preferred Stock,
           Series J Preferred Stock, Series K Preferred Stock, Series L
           Preferred Stock or Series M Preferred Stock, dividends shall
           simultaneously be declared and paid on the outstanding shares of
           Series N Preferred Stock, pari passu with the shares of Series D
           Preferred Stock, Series E Preferred Stock, Series F Preferred Stock,
           Series G Preferred Stock, Series H Preferred Stock, Series I
           Preferred Stock, Series J Preferred Stock, Series K Preferred Stock,
           Series L Preferred Stock and Series M Preferred Stock, based upon the
           number of shares of Common Stock into which shares of Series D
           Preferred Stock, Series E Preferred Stock, Series F Preferred Stock,
           Series G Preferred Stock, Series H Preferred Stock, Series I
           Preferred Stock, Series J Preferred Stock, Series K Preferred Stock,
           Series L Preferred Stock, Series M Preferred Stock and Series N
           Preferred Stock are then convertible. In the event the Corporation
           shall at any time after the date of the filing of this Certificate of
           Determination of Preferences (the "Rights Declaration Date") (i)
           declare any dividend on Common Stock payable in shares of Common
           Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine
           the outstanding Common Stock into a smaller number of shares, then in
           each such case, the amount of Common Stock or other consideration to
           which holders of shares of Series N Preferred Stock were entitled
           immediately prior to such event under the preceding sentence shall be
           adjusted as set forth in Section 4(C) hereof.

                               (B) The Corporation shall declare a dividend or
           distribution on the Series N Preferred Stock as provided in paragraph
           (A) above prior to declaring a dividend payable on shares of Common
           Stock.

                     Section 3.  Voting Rights.  The holders of shares of
           Series N Preferred Stock shall have the following voting rights:

                               (A) Each holder of Series N Preferred Stock is
           entitled to a number of votes equal to the number of shares of Common
           Stock into which the holder's Series N Preferred Stock is then
           convertible. Except as provided by law, the Common Stock and Series N
           Preferred Stock (and any series of Preferred Stock which may be
           subsequently authorized which is convertible into shares of Common
           Stock and which has voting rights equal to the number of shares of
           Common Stock into which such series of Preferred Stock is then
           convertible) shall vote together as a single class on all matters to
           come before the shareholders for approval. In the event the
           Corporation shall at any time after the Rights Declaration Date (i)
           declare any dividend on Common Stock payable in shares of Common
           Stock, (ii) subdivide the outstanding Common Stock, or (iii) combine
           the outstanding Common Stock into a smaller number of shares, then in
           each such case the number of votes per share to which holders of
           shares of Series N Preferred Stock were entitled immediately prior to
           such event shall be adjusted as set forth in Section 4(C) hereof.





                                       56
<PAGE>   3

                               (B) Except as otherwise provided herein or by
           law, the holders of shares of Series N Preferred Stock and the
           holders of shares of Common Stock (and any series of Preferred Stock
           which may be subsequently authorized which is convertible into shares
           of Common Stock and which has voting rights equal to the number of
           shares of Common Stock into which such series of Preferred Stock is
           then convertible) shall vote together as one class on all matters
           submitted to a vote of shareholders of the Corporation.

                               (C) Except as required by law or under Section 8
           hereof, holders of Series N Preferred Stock shall have no special
           voting rights and their consent shall not be required (except to the
           extent they are entitled to vote with holders of Common Stock as set
           forth herein) for taking any corporate action.

                     Section 4.  Conversion Rights.

                               (A) Each holder of Series N Preferred Stock may,
           at any time, in such holder's sole discretion, convert all or any
           part of such holder's shares of Series N Preferred Stock into fully
           paid and nonassessable shares of Common Stock at the rate of one (1)
           share of Common Stock for each share of Series N Preferred Stock
           surrendered for conversion.

                               (B) Such conversion may be effected by surrender
           of such holder's certificate or certificates for the shares of Series
           N Preferred Stock to be converted, duly endorsed, at the principal
           office of the Corporation, with a written notice stating (i) that
           such holder elects to convert all or a specified number of shares of
           Series N Preferred Stock into shares of Common Stock, and (ii) the
           name in which such holder desires a certificate for the shares of
           Common Stock to be issued. Promptly thereafter, the Company shall
           issue and deliver to such holder a certificate for the number of
           shares of Common Stock to which such holder shall be entitled. Such
           conversion shall be deemed to have been made at the close of business
           on the date of such surrender, and such holder shall be treated for
           all purposes as the record holder of such shares of Common Stock on
           that date.

                               (C) In the event the Corporation shall at any
           time after the Rights Declaration Date (i) declare any dividend on
           Common Stock payable in shares of Common Stock, (ii) subdivide the
           outstanding Common Stock, or (iii) combine the outstanding Common
           Stock into a smaller number of shares, then, in each case, the number
           of shares of Common Stock issuable upon the conversion of each share
           of Series N Preferred Stock shall be adjusted by multiplying such
           amount by a fraction, the numerator of which is the number of shares
           of Common Stock outstanding immediately after such event and the
           denominator of which is the number of shares of Common Stock that are
           outstanding immediately prior to such event.

                               (D) In the event the Corporation shall at any
           time or from time to time after the Rights Declaration Date make or
           issue, or fix a record date for the determination of holders of
           Common Stock entitled to receive, a dividend or other distribution
           payable in securities of the Corporation or any of its subsidiaries,
           or of any other corporation or third




                                       57
<PAGE>   4

           party, other than in shares of Common Stock, then, in each such
           event, provisions shall be made so that the holders of Series N
           Preferred Stock shall receive, upon the conversion thereof,
           securities of the Corporation or any of its subsidiaries or of any
           other corporation or third party which they would have received had
           their stock been converted into Common Stock on the date of such
           event.

                     Section 5.  Reacquired Shares. Any shares of Series N
           Preferred Stock purchased or otherwise acquired by the Corporation in
           any manner whatsoever shall be retired and canceled promptly after
           the acquisition thereof. All such shares shall upon their
           cancellation become authorized but unissued shares of Preferred Stock
           and may be reissued as part of a new series of Preferred Stock to be
           created by resolution or resolutions of the Board of Directors,
           subject to the conditions and restrictions on issuance set forth
           herein.

                     Section 6.  Liquidation, Dissolution or Winding Up.

                               (A) Upon any liquidation (voluntary or
           otherwise), dissolution or winding up of the Corporation, following
           the first priority liquidation preference of the Series C Preferred
           Stock in the amount of $5.00 per share plus any declared but unpaid
           dividends, and pari passu with the liquidation preference of the
           Series D Preferred Stock in the amount of $5.00 per share plus any
           declared but unpaid dividends ("Series D Liquidation Preference"),
           the liquidation preference of the Series E Preferred Stock in the
           amount of $4.00 per share plus any declared but unpaid dividends
           ("Series E Liquidation Preference"), the liquidation preference of
           the Series F Preferred Stock in the amount of $8.00 per share plus
           any declared but unpaid dividends ("Series F Liquidation
           Preference"), the liquidation preference of the Series G Preferred
           Stock in the amount of $20.00 per share plus any declared but unpaid
           dividends ("Series G Liquidation Preference"), the liquidation
           preference of the Series H Preferred Stock in the amount of $38.30
           per share plus any declared but unpaid dividends ("Series H
           Liquidation Preference"), the liquidation preference of the Series I
           Preferred Stock in the amount of $4.19 per share plus any declared
           but unpaid dividends ("Series I Liquidation Preference"), the
           liquidation preference of the Series J Preferred Stock in the amount
           of $2.98 per share plus any declared but unpaid dividends ("Series J
           Liquidation Preference"), the liquidation preference of the Series K
           Preferred Stock in the amount of $3.19 per share plus any declared
           but unpaid dividends ("Series K Liquidation Preference"), the
           liquidation preference of the Series L Preferred Stock in the amount
           of $2.76 per share plus any declared but unpaid dividends ("Series L
           Liquidation Preference") and the liquidation preference of the Series
           M Preferred Stock in the amount of $2.23 per share plus any declared
           but unpaid dividends ("Series M Liquidation Preference") no
           distribution shall be made to the holders of shares of stock ranking
           junior (either as to dividends or upon liquidation, dissolution or
           winding up) to the Series N Preferred Stock unless, prior thereto,
           the holders of shares of Series N Preferred Stock shall have received
           an amount equal to $.90 per share of Series N Preferred Stock plus
           any declared but unpaid dividends ("Series N Liquidation
           Preference").





                                       58
<PAGE>   5


                               (B) In the event, however, that there are not
           sufficient assets available to permit payment in full of the Series D
           Liquidation Preference, the Series E Liquidation Preference, the
           Series F Liquidation Preference, the Series G Liquidation Preference,
           the Series H Liquidation Preference, the Series I Liquidation
           Preference, the Series J Liquidation Preference, the Series K
           Liquidation Preference, the Series L Liquidation Preference, the
           Series M Liquidation Preference, the Series N Liquidation Preference
           and the liquidation preferences of all other series of Preferred
           Stock, if any, which rank on a parity with the Series N Preferred
           Stock, then such remaining assets shall be distributed ratably to the
           holders of such parity shares in proportion to their respective
           liquidation preferences.

                     Section 7.  No Redemption.  The shares of Series N
           Preferred Stock shall not be redeemable.

                     Section 8. Amendment. The Articles of Incorporation of the
           Corporation shall not be further amended in any manner which would
           (a) alter or change the powers, preferences or special rights or
           privileges of the Series N Preferred Stock so as to affect them
           adversely or (b) grant to any other class of shares any rights
           superior to those of the Series N Preferred Stock without the
           affirmative vote of the holders of a majority or more of the
           outstanding shares of Series N Preferred Stock, voting separately as
           a class.

                     Section 9. Fractional Shares. Series N Preferred Stock may
           be issued in fractions of a share which shall entitle the holder, in
           proportion to such holder's fractional shares, to exercise voting
           rights, receive dividends, participate in distributions and to have
           the benefit of all other rights of holders of Series N Preferred
           Stock."





                                       59
<PAGE>   6

           The undersigned declare under penalty of perjury that the matters set
forth in the foregoing Certificate are true of their own knowledge.

           Executed at Milpitas, California on January 5, 1998.



                                        /s/ J. Richard Ellis
                                        -------------------------------------
                                        J. Richard Ellis
                                        President and Chief Executive Officer



                                        /s/ Henry Madrid
                                        -------------------------------------
                                        Henry Madrid
                                        Chief Financial Officer








                                       60